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/realise.md

@@ -0,0 +1,118 @@
+# Name
+
+`nix-store --realise` - realise specified store paths
+
+# Synopsis
+
+`nix-store` {`--realise` | `-r`} *paths…* [`--dry-run`]
+
+# Description
+
+The operation `--realise` essentially “builds” the specified store
+paths. Realisation is a somewhat overloaded term:
+
+  - If the store path is a *derivation*, realisation ensures that the
+    output paths of the derivation are [valid] (i.e.,
+    the output path and its closure exist in the file system). This
+    can be done in several ways. First, it is possible that the
+    outputs are already valid, in which case we are done
+    immediately. Otherwise, there may be [substitutes]
+    that produce the outputs (e.g., by downloading them). Finally, the
+    outputs can be produced by running the build task described
+    by the derivation.
+
+  - If the store path is not a derivation, realisation ensures that the
+    specified path is valid (i.e., it and its closure exist in the file
+    system). If the path is already valid, we are done immediately.
+    Otherwise, the path and any missing paths in its closure may be
+    produced through substitutes. If there are no (successful)
+    substitutes, realisation fails.
+
+[valid]: @docroot@/glossary.md#gloss-validity
+[substitutes]: @docroot@/glossary.md#gloss-substitute
+
+The output path of each derivation is printed on standard output. (For
+non-derivations argument, the argument itself is printed.)
+
+The following flags are available:
+
+  - `--dry-run`\
+    Print on standard error a description of what packages would be
+    built or downloaded, without actually performing the operation.
+
+  - `--ignore-unknown`\
+    If a non-derivation path does not have a substitute, then silently
+    ignore it.
+
+  - `--check`\
+    This option allows you to check whether a derivation is
+    deterministic. It rebuilds the specified derivation and checks
+    whether the result is bitwise-identical with the existing outputs,
+    printing an error if that’s not the case. The outputs of the
+    specified derivation must already exist. When used with `-K`, if an
+    output path is not identical to the corresponding output from the
+    previous build, the new output path is left in
+    `/nix/store/name.check.`
+
+Special exit codes:
+
+  - `100`\
+    Generic build failure, the builder process returned with a non-zero
+    exit code.
+
+  - `101`\
+    Build timeout, the build was aborted because it did not complete
+    within the specified `timeout`.
+
+  - `102`\
+    Hash mismatch, the build output was rejected because it does not
+    match the [`outputHash` attribute of the
+    derivation](@docroot@/language/advanced-attributes.md).
+
+  - `104`\
+    Not deterministic, the build succeeded in check mode but the
+    resulting output is not binary reproducible.
+
+With the `--keep-going` flag it's possible for multiple failures to
+occur, in this case the 1xx status codes are or combined using binary
+or.
+
+    1100100
+       ^^^^
+       |||`- timeout
+       ||`-- output hash mismatch
+       |`--- build failure
+       `---- not deterministic
+
+
+{{#include ./opt-common.md}}
+
+{{#include ../opt-common.md}}
+
+{{#include ../env-common.md}}
+
+# Examples
+
+This operation is typically used to build [store derivation]s produced by
+[`nix-instantiate`](@docroot@/command-ref/nix-instantiate.md):
+
+[store derivation]: @docroot@/glossary.md#gloss-store-derivation
+
+```console
+$ nix-store -r $(nix-instantiate ./test.nix)
+/nix/store/31axcgrlbfsxzmfff1gyj1bf62hvkby2-aterm-2.3.1
+```
+
+This is essentially what [`nix-build`](@docroot@/command-ref/nix-build.md) does.
+
+To test whether a previously-built derivation is deterministic:
+
+```console
+$ nix-build '<nixpkgs>' -A hello --check -K
+```
+
+Use [`nix-store --read-log`](./read-log.md) to show the stderr and stdout of a build:
+
+```console
+$ nix-store --read-log $(nix-instantiate ./test.nix)
+```