Add options and parts to frontmatter

[fwkoch]

• options are template options for site config or page/project frontmatter; this (1) allows options to be provided in a centralized place, rather than on each export, and (2) gives an explicit place for these arbitrary options to be found
  • TODO: tidy up warning messages for unused options, now that options may apply to different exports, so they may be unused on one place but used in another.
• parts are document parts, previously only defined with block metadata part: 'abstract', now you can put the abstract directly in the frontmatter. It is then inserted into the document as a hidden block, and transformed with the rest of the content.
  • TODO: audit exports to make sure hidden: true visibility: 'remove' is respected on blocks - This is the case for exports defined in mystmd.
  • For site builds, I added this pr: 🧱 Respect visibility: remove on blocks myst-theme#259

[rowanc1]

Some comments!

[rowanc1]

I think merging #741 would be good. We need to by default turn the extracted abstracts, etc. to visible.

[rowanc1]

Have tested parts and that works great! Still need to test out options.

[stevejpurves]

@fwkoch do you have an example myst.yml file that you can post here showing the layout of parts and options after this PR?