Changes

Minor CSS fixup for glossary link
Upgrade to crate-docs 2.0.0
Permit installation on Sphinx 4
Add intersphinx mapping for cross referencing documentation across different repositories
Improve bundling of custom JavaScript and CSS assets
Fix the docs title for Segment.io / GA tracking
Add ESI snippets for a dynamic promotion header and newsletter footer
Improve margins and rename section class to w-canvas for proper tagging
Add extension "sphinxcontrib.plantuml"
Add extension "sphinxext.opengraph"
Add "sphinx-copybutton" extension
Use Pygments style "material" for code highlighting
Add doing_docs config module (hidden menu item)

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