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-shell.md

@@ -101,7 +101,7 @@ All options not listed here are passed to `nix-store
     When a `--pure` shell is started, keep the listed environment
     variables.
 
-The following common options are supported:
+{{#include ./opt-common.md}}
 
 # Environment variables
 
@@ -110,6 +110,8 @@ The following common options are supported:
     `bash` found in `<nixpkgs>`, falling back to the `bash` found in
     `PATH` if not found.
 
+{{#include ./env-common.md}}
+
 # Examples
 
 To build the dependencies of the package Pan, and start an interactive