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/opt-common.md

@@ -0,0 +1,35 @@
+# Options
+
+The following options are allowed for all `nix-env` operations, but may not always have an effect.
+
+  - `--file` / `-f` *path*\
+    Specifies the Nix expression (designated below as the *active Nix
+    expression*) used by the `--install`, `--upgrade`, and `--query
+    --available` operations to obtain derivations. The default is
+    `~/.nix-defexpr`.
+
+    If the argument starts with `http://` or `https://`, it is
+    interpreted as the URL of a tarball that will be downloaded and
+    unpacked to a temporary location. The tarball must include a single
+    top-level directory containing at least a file named `default.nix`.
+
+  - `--profile` / `-p` *path*\
+    Specifies the profile to be used by those operations that operate on
+    a profile (designated below as the *active profile*). A profile is a
+    sequence of user environments called *generations*, one of which is
+    the *current generation*.
+
+  - `--dry-run`\
+    For the `--install`, `--upgrade`, `--uninstall`,
+    `--switch-generation`, `--delete-generations` and `--rollback`
+    operations, this flag will cause `nix-env` to print what *would* be
+    done if this flag had not been specified, without actually doing it.
+
+    `--dry-run` also prints out which paths will be
+    [substituted](@docroot@/glossary.md) (i.e., downloaded) and which paths
+    will be built from source (because no substitute is available).
+
+  - `--system-filter` *system*\
+    By default, operations such as `--query
+                    --available` show derivations matching any platform. This option
+    allows you to use derivations for the specified platform *system*.