Add go module for use with new Hugo docs UI

[nmelehan]

This is a prerequisite for generating previews of the API spec with a local copy of the repo. Once merged, these instructions can be followed to generate those local previews:

1. From the feature/new-docs-ui branch of your docs repo, run:

    go mod edit -replace github.com/linode/linode-api-docs=/Users/nmelehan/Desktop/linode/linode-api-docs. 
   

   Be sure to replace the local file path of the linode-api-docs repo with the one from your computer.

2. Run the hugo server command with the --ignoreVendor option:

    hugo server --ignoreVendor
   

   The local server should now live-reload with any changes you make to your local API docs repo.

3. When you previously ran the go mod edit -replace command, a line was added to your docs repo’s go.mod file that looks like this:

    replace github.com/linode/linode-api-docs => /Users/nmelehan/Desktop/linode/linode-api-docs. 
   

   You should remove this line from that file when you are done testing the API locally.