Remove documentation site and GitHub Action, update README references

[ADubhlaoich]

Proposed changes

As of late December, NGINX has open sourced its documentation repository. The next stage of this effort is to consolidate all of the existing documentation into the single repository.

This commit removes the entire /site folder that was being used to manage the documentation for NGINX Gateway Fabric separately from the main pipeline, as well as the GitHub action used for deployment.

It also updates references to the folder to instead point towards the documentation repository, where all updates will be made in the future.

Paired with nginx/documentation#81

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING doc
• I have added tests that prove my fix is effective or that my feature works
• I have checked that all unit tests pass after adding my changes
• I have updated necessary documentation
• I have rebased my branch onto main
• I will ensure my PR is targeting the main branch and pulling from my branch from my own fork

Release notes

If this PR introduces a change that affects users and needs to be mentioned in the release notes,
please add a brief note that summarizes the change.

Migrated the documentation website into the NGINX documentation repository.

[ADubhlaoich]

Throwing this up as a draft to ensure nobody fires the gun on this early - the main repository PR to migrate this content hasn't been merged, and I will not put this out of draft until then.