Building the reference database from a template

[Michael Orlitzky]

Obligatory heads-up: I have a PR open at

https://github.com/sagemath/sage/pull/42447

that moves the long list of references to a dedicated, unformatted
file in src/doc. The formatted index.rst is then generated by a
script, from a template, as part of the docbuild.

The benefit is that the script can check for duplicate labels, syntax
errors, and sorting early on. It also lets us put the database in a
more discoverable location.

I am posting about it here because it will move something important,
but there is also a fun opportunity to bikeshed the sort order
enforced by the script. I have it case-insensitive within each letter
group now, but personally don't care what we end up with.