Doc: update compiling.rst

* Emphasize 3.12.1 requirement
* Check for 3.12.1 and run `opam switch install` when not available
Doc: fix broken link to PyNAST
Merge pull request #321 from matsen/321-ocaml-version-in-docs

Clarify OCaml 3.12.1 requirement
Merge branch 'master' into dev; fixing conflicts.

Conflicts:
	docs/compiling.rst
Cleaning out cruft.