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.
* Introduce concretizer:unify option to replace spack:concretization
* Deprecate concretization
* Make spack:concretization overrule concretize:unify for now
* Add environment update logic to move from spack:concretization to spack:concretizer:reuse
* Migrate spack:concretization to spack:concretize:unify in all locations
* For new environments make concretizer:unify explicit, so that defaults can be changed in 0.19
This PR removes a few outdated sections from the "Basics" part of the
documentation. It also makes a few topic under the environment section
more prominent by removing an unneeded spack.yaml subsection and
promoting everything under it.
