Updates Upgrade ECK page

[kosabogi]

Preview

This PR updates the Upgrade Elastic Cloud on Kubernetes page.

Related issue: https://github.com/elastic/docs-projects/issues/460

[eedugon]

We cannot assume the content of this document is valid, as we don't know the details about how to upgrade to 3.x.

Actually with the implemented changes (replacing occurrences of 2.16 with 3.0) the content is probably incorrect, as what was a minor upgrade in the past (2.0 towards 2.x) is now a major upgrade and we are stating:

There are no special instructions to follow if you upgrade from any 2.x version to 3.0.0. Use the upgrade method applicable to your installation method of choice.

We don't know if the previous is valid (might be, but might not be).

I think we need to either involve devs here to help with the upgrade to 3.0 process or just merge this document and devs will refine at a later stage. @pebrc , @barkbay , what do you think?

This PR just adds the ECK upgrade document migrated to markdown to the new Information Architecture (IA): https://docs-v3-preview.elastic.dev/elastic/docs-content/pull/702/deploy-manage/upgrade/orchestrator/upgrade-cloud-on-k8s

We have applied very small changes replacing 2.16 with 3.0.0. Let us know how you would like to handle this.

[eedugon]

Everything looks good in terms of structure and format.
I just raised a comment about a sentence to improve.

after merging we need to let ECK devs know that the upgrade to 3.0.0 doc is this one. They will probably need to update the content.

[barkbay]

LGTM

There are no major technical changes in version 3.0, so I think the upgrade documentation can remain unchanged.

[eedugon]

Thanks @barkbay , then we can merge it and refine at a later stage if needed.