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-store/gc.md

@@ -0,0 +1,72 @@
+# Name
+
+`nix-store --gc` - run garbage collection
+
+# Synopsis
+
+`nix-store` `--gc` [`--print-roots` | `--print-live` | `--print-dead`] [`--max-freed` *bytes*]
+
+# Description
+
+Without additional flags, the operation `--gc` performs a garbage
+collection on the Nix store. That is, all paths in the Nix store not
+reachable via file system references from a set of “roots”, are deleted.
+
+The following suboperations may be specified:
+
+  - `--print-roots`\
+    This operation prints on standard output the set of roots used by
+    the garbage collector.
+
+  - `--print-live`\
+    This operation prints on standard output the set of “live” store
+    paths, which are all the store paths reachable from the roots. Live
+    paths should never be deleted, since that would break consistency —
+    it would become possible that applications are installed that
+    reference things that are no longer present in the store.
+
+  - `--print-dead`\
+    This operation prints out on standard output the set of “dead” store
+    paths, which is just the opposite of the set of live paths: any path
+    in the store that is not live (with respect to the roots) is dead.
+
+By default, all unreachable paths are deleted. The following options
+control what gets deleted and in what order:
+
+  - `--max-freed` *bytes*\
+    Keep deleting paths until at least *bytes* bytes have been deleted,
+    then stop. The argument *bytes* can be followed by the
+    multiplicative suffix `K`, `M`, `G` or `T`, denoting KiB, MiB, GiB
+    or TiB units.
+
+The behaviour of the collector is also influenced by the
+`keep-outputs` and `keep-derivations` settings in the Nix
+configuration file.
+
+By default, the collector prints the total number of freed bytes when it
+finishes (or when it is interrupted). With `--print-dead`, it prints the
+number of bytes that would be freed.
+
+{{#include ./opt-common.md}}
+
+{{#include ../opt-common.md}}
+
+{{#include ../env-common.md}}
+
+# Examples
+
+To delete all unreachable paths, just do:
+
+```console
+$ nix-store --gc
+deleting `/nix/store/kq82idx6g0nyzsp2s14gfsc38npai7lf-cairo-1.0.4.tar.gz.drv'
+...
+8825586 bytes freed (8.42 MiB)
+```
+
+To delete at least 100 MiBs of unreachable paths:
+
+```console
+$ nix-store --gc --max-freed $((100 * 1024 * 1024))
+```
+