Oolite 1.93.1-254

Build the documentation with typst only, and make it optional (#792)

The documentation build added in #784 needed pandoc, WeasyPrint (with
Python, Pango and a .venv in the source tree) and typst. Distro source
builds such as the AUR oolite-git package failed without the .venv, and
the dependencies were too heavy to force on users. typst alone can turn
the Markdown pages into PDFs through the cmarker package, as it already
does for the reference sheet.

- docs/typst/markdown.typ renders a Markdown page with the same look
  for <kbd>, page breaks, no-print blocks and attr_list image widths.
  cmarker 0.1.10 (MIT) is vendored in docs/typst/vendor so builds need
  no network access.
- Add a 'docs' meson feature option (default auto): the PDFs are built
  when typst 0.15+ is found and skipped otherwise. CI and the Flatpak
  manifest pass -Ddocs=enabled so release packages always have them.
  install.sh copes with there being no PDFs.
- Remove the pandoc and WeasyPrint install scripts, the Flatpak
  modules and the Pango packages. mkdocs uses a .venv only if there
  is one, activated in build_mkdocs.sh.
- Keep the typst sources out of the ReadTheDocs site.

Fixes #791

Co-authored-by: David Hobley <david.hobley@sharpblue.com.au>