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/serve.md

@@ -0,0 +1,38 @@
+# Name
+
+`nix-store --serve` - serve local Nix store over SSH
+
+# Synopsis
+
+`nix-store` `--serve` [`--write`]
+
+# Description
+
+The operation `--serve` provides access to the Nix store over stdin and
+stdout, and is intended to be used as a means of providing Nix store
+access to a restricted ssh user.
+
+The following flags are available:
+
+  - `--write`\
+    Allow the connected client to request the realization of
+    derivations. In effect, this can be used to make the host act as a
+    remote builder.
+
+{{#include ./opt-common.md}}
+
+{{#include ../opt-common.md}}
+
+{{#include ../env-common.md}}
+
+# Examples
+
+To turn a host into a build server, the `authorized_keys` file can be
+used to provide build access to a given SSH public key:
+
+```console
+$ cat <<EOF >>/root/.ssh/authorized_keys
+command="nice -n20 nix-store --serve --write" ssh-rsa AAAAB3NzaC1yc2EAAAA...
+EOF
+```
+