Create single source of truth for in-tree tags documentation

[mirekdlugosz]

Right now, in-tree tags documentation is duplicated in Readme.md and each tag source file docstring. As is usually a case for duplicated content, of course it has diverged over time

• There are 18 or 19 tags in tree (depending if you count "generic" as real tag), but 13 listed in Readme. Which means that 5 tags are hard to discover and require reading source file to understand how they work
• docstrings often do better work at documentation, by providing more examples and example HTML output
• sometimes Readme provides better documentation, like in youtube tag (where Readme explains YOUTUBE_THUMB_ONLY and YOUTUBE_THUMB_SIZE variables)

This issue can be closed when we have single source of truth for all tags documentation, and that documentation is up to date.

Personally I would explore generating all docs from docstrings using sphinx, and creating readthedocs account to store that, but I am open to other solutions, too.

[justinmayer]

@mirekdlugosz: Many thanks for submitting this issue. I just realized that Graphviz is a supported tag, even though it's not mentioned in the README, and then I came across this issue pointing out the general tag documentation disconnect.

I agree that it would be preferable to have the documentation up-to-date and available in single-source-of-truth form. Would anyone like to help out by starting the documentation process? I suggest we use a combination of Sphinx + MyST + Furo (latter is a modern Sphinx theme that supports and recommends MyST).

What do you think? (cc: @dorneanu @Xion @rlaboiss)