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/generate-binary-cache-key.md

@@ -0,0 +1,29 @@
+# Name
+
+`nix-store --generate-binary-cache-key` - generate key pair to use for a binary cache
+
+## Synopsis
+
+`nix-store` `--generate-binary-cache-key` *key-name* *secret-key-file* *public-key-file*
+
+## Description
+
+This command generates an [Ed25519 key pair](http://ed25519.cr.yp.to/)
+that can be used to create a signed binary cache. It takes three
+mandatory parameters:
+
+1.  A key name, such as `cache.example.org-1`, that is used to look up
+    keys on the client when it verifies signatures. It can be anything,
+    but it’s suggested to use the host name of your cache (e.g.
+    `cache.example.org`) with a suffix denoting the number of the key
+    (to be incremented every time you need to revoke a key).
+
+2.  The file name where the secret key is to be stored.
+
+3.  The file name where the public key is to be stored.
+
+{{#include ./opt-common.md}}
+
+{{#include ../opt-common.md}}
+
+{{#include ../env-common.md}}