Split nix-env and nix-store documentation per-subcommand

Documentation on "classic" commands with many sub-commands are
notoriously hard to discover due to lack of overview and anchor links.
Additionally the information on common options and environment variables
is not accessible offline in man pages, and therefore often overlooked
by readers.

With this change, each sub-command of nix-store and nix-env gets its
own page in the manual (listed in the table of contents), and each own
man page.

Also, man pages for each subcommand now (again) list common options
and environment variables. While this makes each page quite long and
some common parameters don't apply, this should still make it easier
to navigate as that additional information was not accessible on the
command line at all.

It is now possible to run 'nix-store --<subcommand> --help` to display
help pages for the given subcommand.

Co-authored-by: Valentin Gagarin <valentin.gagarin@tweag.io>

doc/manual/src/command-ref/nix-env/switch-generation.md

@@ -0,0 +1,33 @@
+# Name
+
+`nix-env --switch-generation` - set user environment to given profile generation
+
+# Synopsis
+
+`nix-env` {`--switch-generation` | `-G`} *generation*
+
+# Description
+
+This operation makes generation number *generation* the current
+generation of the active profile. That is, if the `profile` is the path
+to the active profile, then the symlink `profile` is made to point to
+`profile-generation-link`, which is in turn a symlink to the actual user
+environment in the Nix store.
+
+Switching will fail if the specified generation does not exist.
+
+{{#include ./opt-common.md}}
+
+{{#include ../opt-common.md}}
+
+{{#include ./env-common.md}}
+
+{{#include ../env-common.md}}
+
+# Examples
+
+```console
+$ nix-env -G 42
+switching from generation 50 to 42
+```
+