build(docs): set `site_url` from ReadTheDocs env var

[agilgur5]

Motivation

• site_url is required for canonicalization, which is a prereq for proper styling on 404s and creating a sitemap.xml

• Also for certain features like:

  • Instant loading
  • Versioning (we use RTD's versioning, but may want some features from here like the version drop-down)

Modifications

• Add site_url config to mkdocs.yml
  • use RTD env var

Verification

1. mkdocs serve
2. Go to http://127.0.0.1:8000/en/latest/ now -- works
3. Go to http://127.0.0.1:8000/en/latest/sitemap.xml -- actually has content now
4. Go to http://127.0.0.1:8000/en/latest/wrong -- has a properly styled 404 page.
   Screenshot:

   

[agilgur5]

• site_url is required for canonicalization and creating a sitemap.xml
• Also for certain features like:

Ah I think site_url was also required for to make 404 links (like https://argo-workflows.readthedocs.io/en/release-3.4/wrong) render CSS properly, and that was the original purpose of this PR, but I've forgotten what links I followed (went down a rabbit hole at the time). https://github.com/mkdocs/mkdocs/issues/2318 is related

[agilgur5]

Ah I think site_url was also required for to make 404 links (like https://argo-workflows.readthedocs.io/en/release-3.4/wrong) render CSS properly, and that was the original purpose of this PR, but I've forgotten what links I followed (went down a rabbit hole at the time).

Found it again: https://docs.readthedocs.io/en/stable/reference/404-not-found.html, under the MkDocs tab:

MkDocs automatically generates a 404.html which Read the Docs will use. However, assets will not be loaded correctly unless you define the site_url configuration value as your site’s canonical base URL.