Use sphinx-design to format tags as pills/buttons

[melissawm]

See https://sphinx-design.readthedocs.io/en/latest/badges_buttons.html

[nebelgrau77]

I really like the buttons! But some questions come to mind:

• does this feature require sphinx-design extension? I'm using the RTD theme and it seems to be working out of the box, but maybe it only does with those specific themes mentioned in the link above (RTD, Alabaster and some others)?
• would the sphinx_tags extension ideally turn tags into these buttons (or badges), i.e. instead of adding Tags: tag1, tag2 in a tag file, add Tags: :bdg:`tag1`, :bdg-info:`tag2`? if that is the case, we could choose one style, say, info, but then how could the user choose different styles? I guess we would either need to stick to just one (and it could be selectable in the conf.py), or as an alternative cycle through the seven colors (primary/secondary/info/success... etc.)

[JWCook]

I'd really like to see this feature too.

but then how could the user choose different styles

It would probably make the most sense to put that info in config. For example, a mapping from tag names to colors:

tag_colors= {
    'tag_1': 'primary',
    'tag_2': 'info',
    ...
}

[JWCook]

An incremental improvement to that would be glob patterns:

tag_colors = {
    'tag_2':     'info',
    'prefix:*':  'warning',
    '*':         'light',  # default role for any tags that don't one of these patterns
}

I have some changes for this in #35. That's just one way to do it, though, so I'm open to suggestions.