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-collect-garbage.md

@@ -9,7 +9,7 @@
 # Description
 
 The command `nix-collect-garbage` is mostly an alias of [`nix-store
---gc`](nix-store.md#operation---gc), that is, it deletes all
+--gc`](@docroot@/command-ref/nix-store/gc.md), that is, it deletes all
 unreachable paths in the Nix store to clean up your system. However,
 it provides two additional options: `-d` (`--delete-old`), which
 deletes all old generations of all profiles in `/nix/var/nix/profiles`
@@ -20,6 +20,10 @@ and `--delete-older-than` *period*, where period is a value such as
 of days in all profiles in `/nix/var/nix/profiles` (except for the
 generations that were active at that point in time).
 
+{{#include ./opt-common.md}}
+
+{{#include ./env-common.md}}
+
 # Example
 
 To delete from the Nix store everything that is not used by the current