* 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This PR removes a few outdated sections from the "Basics" part of the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.
