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/verify.md

@@ -0,0 +1,36 @@
+# Name
+
+`nix-store --verify` - check Nix database for consistency
+
+# Synopsis
+
+`nix-store` `--verify` [`--check-contents`] [`--repair`]
+
+# Description
+
+The operation `--verify` verifies the internal consistency of the Nix
+database, and the consistency between the Nix database and the Nix
+store. Any inconsistencies encountered are automatically repaired.
+Inconsistencies are generally the result of the Nix store or database
+being modified by non-Nix tools, or of bugs in Nix itself.
+
+This operation has the following options:
+
+  - `--check-contents`\
+    Checks that the contents of every valid store path has not been
+    altered by computing a SHA-256 hash of the contents and comparing it
+    with the hash stored in the Nix database at build time. Paths that
+    have been modified are printed out. For large stores,
+    `--check-contents` is obviously quite slow.
+
+  - `--repair`\
+    If any valid path is missing from the store, or (if
+    `--check-contents` is given) the contents of a valid path has been
+    modified, then try to repair the path by redownloading it. See
+    `nix-store --repair-path` for details.
+
+{{#include ./opt-common.md}}
+
+{{#include ../opt-common.md}}
+
+{{#include ../env-common.md}}