ENH: Add MyST support and tests for ref labels

[JWCook]

Closes #85; Follow-up from #86

Notes:

• Added explicit ref label for MyST tag pages so the same label can be used for either format
• Added ref label above page header, so ref links use the header as the default label instead of the toctree title (e.g., "Tag: tag_1" instead of "pages with this tag")
• On quickstart page, added tabs to switch between usage examples for rST and MyST
• Unrelated change: Pinned minimum version of pydata-sphinx-theme due to UndefinedError("'logo' is undefined") errors. See: https://github.com/pydata/pydata-sphinx-theme/issues/1094

[JWCook]

@lucyleeow Does this look reasonable to you?