Experiment adding a dynamic canonical URL to mkdocs

[benjaoming]

This PR is currently stuck, looking for a way to use !ENV in .mkdocs

[benjaoming]

This doesn't work because of yaml_load_safely

site_url is set to null:

 cat mkdocs.yml
docs_dir: docs
extra_css:
- /_/static/css/badge_only.css
- /_/static/css/readthedocs-doc-embed.css
extra_javascript:
- readthedocs-data.js
- /_/static/core/js/readthedocs-doc-embed.js
- /_/static/javascript/readthedocs-analytics.js
google_analytics: null
markdown_extensions:
- markdown_include.include:
    base_path: .
- admonition
plugins:
- search
- mkdocstrings:
    handlers:
      python:
        options:
          docstring_style: sphinx
site_name: Basic MkDocs Example Project
site_url: null
theme:
  highlightjs: true
  name: readthedocs 

[benjaoming]

Blocked by readthedocs/readthedocs.org#8529