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/add-fixed.md

@@ -0,0 +1,35 @@
+# Name
+
+`nix-store --add-fixed` - add paths to store using given hashing algorithm
+
+## Synopsis
+
+`nix-store` `--add-fixed` [`--recursive`] *algorithm* *paths…*
+
+## Description
+
+The operation `--add-fixed` adds the specified paths to the Nix store.
+Unlike `--add` paths are registered using the specified hashing
+algorithm, resulting in the same output path as a fixed-output
+derivation. This can be used for sources that are not available from a
+public url or broke since the download expression was written.
+
+This operation has the following options:
+
+  - `--recursive`\
+    Use recursive instead of flat hashing mode, used when adding
+    directories to the store.
+
+{{#include ./opt-common.md}}
+
+{{#include ../opt-common.md}}
+
+{{#include ../env-common.md}}
+
+## Example
+
+```console
+$ nix-store --add-fixed sha256 ./hello-2.10.tar.gz
+/nix/store/3x7dwzq014bblazs7kq20p9hyzz0qh8g-hello-2.10.tar.gz
+```
+