Enable link checker in CI and fix broken links

[hasbro17]

Added a travis job to run htmlproofer on the website docs to check for broken links.

Additionally fixed broken links.

[anik120]

/lgtm
thank you for this PR!

[anik120]

/lgtm

[anik120]

/cherry-pick v0.15.x

[openshift-cherrypick-robot]

@anik120: #49 failed to apply on top of branch "v0.15.x":

Applying: Enable link checker in CI and fix broken links
.git/rebase-apply/patch:41: trailing whitespace.
The `.spec` of a CatalogSource indicates to how to construct a pod or how to talk to a service that serves the [operator-registry grpc api](/docs/concepts/olm-architecture/operator-registry/using-catalog-locally). 
.git/rebase-apply/patch:146: trailing whitespace.
  [Once you've made your operator available in a catalog](/docs/tasks/make-operator-part-of-catalog/), or you've chosen an operator from the [list of operators available to be installed in the cluster](/docs/tasks/list-operators-available-to-install/), you can install your operator by creating a [`Subscription`](/docs/concepts/crds/subscription/) to a specific [channel](/docs/glossary/glossary/#channel). 
.git/rebase-apply/patch:159: trailing whitespace.
  Once you've packaged your operator and made it part of catalog, you can see it among the list of operators that are available to install in the cluster. There is an extension API in OLM named `PackageManifest` that contains information about existing [CatalogSources](/docs/concepts/crds/catalogsource), which is essentially a collection of bundles that each define an operator in the cluster. By querying that API, you can see the list of available operators.  
warning: 3 lines add whitespace errors.
Using index info to reconstruct a base tree...
M	content/en/docs/Concepts/crds/CatalogSource.md
M	content/en/docs/Tasks/install-operator-with-olm.md
M	content/en/docs/Tasks/packaging-an-operator.md
A	content/en/docs/advanced-tasks/operator-scoping-with-operatorgroups.md
Falling back to patching base and 3-way merge...
CONFLICT (modify/delete): content/en/docs/advanced-tasks/operator-scoping-with-operatorgroups.md deleted in HEAD and modified in Enable link checker in CI and fix broken links. Version Enable link checker in CI and fix broken links of content/en/docs/advanced-tasks/operator-scoping-with-operatorgroups.md left in tree.
Auto-merging content/en/docs/Tasks/packaging-an-operator.md
Auto-merging content/en/docs/Tasks/install-operator-with-olm.md
Auto-merging content/en/docs/Concepts/crds/CatalogSource.md
error: Failed to merge in the changes.
hint: Use 'git am --show-current-patch=diff' to see the failed patch
Patch failed at 0001 Enable link checker in CI and fix broken links
When you have resolved this problem, run "git am --continue".
If you prefer to skip this patch, run "git am --skip" instead.
To restore the original branch and stop patching, run "git am --abort".

In response to this:

/cherry-pick v0.15.x

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.

[anik120]

/cherry-pick legacy

[openshift-cherrypick-robot]

@anik120: #49 failed to apply on top of branch "legacy":

Applying: Enable link checker in CI and fix broken links
.git/rebase-apply/patch:41: trailing whitespace.
The `.spec` of a CatalogSource indicates to how to construct a pod or how to talk to a service that serves the [operator-registry grpc api](/docs/concepts/olm-architecture/operator-registry/using-catalog-locally). 
.git/rebase-apply/patch:146: trailing whitespace.
  [Once you've made your operator available in a catalog](/docs/tasks/make-operator-part-of-catalog/), or you've chosen an operator from the [list of operators available to be installed in the cluster](/docs/tasks/list-operators-available-to-install/), you can install your operator by creating a [`Subscription`](/docs/concepts/crds/subscription/) to a specific [channel](/docs/glossary/glossary/#channel). 
.git/rebase-apply/patch:159: trailing whitespace.
  Once you've packaged your operator and made it part of catalog, you can see it among the list of operators that are available to install in the cluster. There is an extension API in OLM named `PackageManifest` that contains information about existing [CatalogSources](/docs/concepts/crds/catalogsource), which is essentially a collection of bundles that each define an operator in the cluster. By querying that API, you can see the list of available operators.  
warning: 3 lines add whitespace errors.
Using index info to reconstruct a base tree...
M	content/en/docs/Concepts/crds/CatalogSource.md
M	content/en/docs/Tasks/install-operator-with-olm.md
M	content/en/docs/Tasks/packaging-an-operator.md
A	content/en/docs/advanced-tasks/operator-scoping-with-operatorgroups.md
Falling back to patching base and 3-way merge...
CONFLICT (modify/delete): content/en/docs/advanced-tasks/operator-scoping-with-operatorgroups.md deleted in HEAD and modified in Enable link checker in CI and fix broken links. Version Enable link checker in CI and fix broken links of content/en/docs/advanced-tasks/operator-scoping-with-operatorgroups.md left in tree.
Auto-merging content/en/docs/Tasks/packaging-an-operator.md
Auto-merging content/en/docs/Tasks/install-operator-with-olm.md
Auto-merging content/en/docs/Concepts/crds/CatalogSource.md
error: Failed to merge in the changes.
hint: Use 'git am --show-current-patch=diff' to see the failed patch
Patch failed at 0001 Enable link checker in CI and fix broken links
When you have resolved this problem, run "git am --continue".
If you prefer to skip this patch, run "git am --skip" instead.
To restore the original branch and stop patching, run "git am --abort".

In response to this:

/cherry-pick legacy

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.