Render PDEPs using myst-parser and the sphinx doc theme

[jorisvandenbossche]

Quick experiment for the discussion in https://github.com/pandas-dev/pandas/issues/51461

• Individual PDEP pages are built with sphinx
• I injected a top navbar in the template mimicking the website's navbar, but for now just with a link back to the previous page. I think it should be possible to let it have the same items/dropdowns as the other pages as well.

Some links to compare current rendering with the one from this PR:

• The roadmap page listing the PDEPs is still the same: https://pandas.pydata.org/preview/51467/about/roadmap.html
• Example PDEP with new rendering: https://pandas.pydata.org/preview/51467/pdeps/0004-consistent-to-datetime-parsing.html (vs https://pandas.pydata.org/pdeps/0004-consistent-to-datetime-parsing.html)