docs: Replace package list with packages.spack.io (#40251)

For a long time, the docs have generated a huge, static HTML package list. It has some
disadvantages:

* It's slow to load
* It's slow to build
* It's hard to search

We now have a nice website that can tell us about Spack packages, and it's searchable so
users can easily find the one or two packages out of 7400 that they're looking for. We
should link to this instead of including a static package list page in the docs.

- [x] Replace package list link with link to packages.spack.io
- [x] Remove `package_list.html` generation from `conf.py`.
- [x] Add a new section for "Links" to the docs.
- [x] Remove docstring notes from contribution guide (we haven't generated RST
      for package docstrings for a while)
- [x] Remove referencese to `package-list` from docs.

lib/spack/docs/build_systems/inteloneapipackage.rst

@@ -25,8 +25,8 @@ use Spack to build packages with the tools.
 The Spack Python class ``IntelOneapiPackage`` is a base class that is
 used by ``IntelOneapiCompilers``, ``IntelOneapiMkl``,
 ``IntelOneapiTbb`` and other classes to implement the oneAPI
-packages. See the :ref:`package-list` for the full list of available
-oneAPI packages or use::
+packages. Search for ``oneAPI`` at `<packages.spack.io>`_ for the full
+list of available oneAPI packages, or use::
 
   spack list -d oneAPI