Docs theme updates

[jputrino]

Proposed changes

This PR changes the configurations for the docs theme. We have moved the theme development to GitHub so it will be accessible in Netlify builds (and the _vendors directory is no longer required). The changes in this PR also let us manage future theme bumps by updating the NGINX_THEME_VERSION variable stored in the Netlify console.

The dev and staging sites are already updated with the changes. Following the merge of this PR, we will need to port the update to the release branch, too.

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

[codecov]

Codecov Report

Merging #3751 (cd7ebf1) into main (082a7c0) will decrease coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #3751      +/-   ##
==========================================
- Coverage   52.35%   52.33%   -0.03%     
==========================================
  Files          59       59              
  Lines       16880    16880              
==========================================
- Hits         8838     8834       -4     
- Misses       7747     7749       +2     
- Partials      295      297       +2     

see 1 file with indirect coverage changes

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[lucacome]

This is awesome! 🚀

[ciarams87]

🤩 🚀