Prep for jupyter releaser

[blink1073]

• Migrate the changelog to markdown and add comment markers for releaser
• Add check release and enforce PR label workflows
• Clean up docs
  -- Switch to pydata sphinx theme
  -- Use myst parser
  -- Build in CI and Handle warnings
• Add link check from maintainer-tools

Used the following command and script for the changelog:

pandoc -f rst -t markdown -o CHANGELOG.md changelog.rst

import re
with open('CHANGELOG.md') as fid:
    data = fid.read()


patt = r'`(\d+)`({.interpreted-text\s+role="ghpull"}:?)'
matches = re.findall(patt, data)


for num, second in matches:
    data = data.replace(f"`{num}`{second}", f"[#{num}](https://github.com/ipython/traitlets/pull/{num})" )


with open("CHANGELOG.md", "w") as fid:
    fid.write(data)