Document file system object content addressing

In addition:

- Take the opportunity to add a bunch more missing hyperlinks, too.

- Remove some glossary entries that are now subsumed by dedicated pages.
  We used to not be able to do this without breaking link fragments, but
  now we can, so pick up where we left off.

Co-authored-by: Robert Hensing <roberth@users.noreply.github.com>

doc/manual/src/command-ref/nix-hash.md

@@ -20,16 +20,21 @@ an example.
 The hash is computed over a *serialisation* of each path: a dump of
 the file system tree rooted at the path. This allows directories and
 symlinks to be hashed as well as regular files. The dump is in the
-*NAR format* produced by [`nix-store
+*[Nix Archive (NAR)][Nix Archive] format* produced by [`nix-store
 --dump`](@docroot@/command-ref/nix-store/dump.md).  Thus, `nix-hash path`
 yields the same cryptographic hash as `nix-store --dump path |
 md5sum`.
 
+[Nix Archive]: @docroot@/store/file-system-object/content-address.md#serial-nix-archive
+
 # Options
 
   - `--flat`\
-    Print the cryptographic hash of the contents of each regular file
-    *path*. That is, do not compute the hash over the dump of *path*.
+    Print the cryptographic hash of the contents of each regular file *path*.
+    That is, instead of computing
+    the hash of the [Nix Archive (NAR)](@docroot@/store/file-system-object/content-address.md#serial-nix-archive) of *path*,
+    just [directly hash]((@docroot@/store/file-system-object/content-address.md#serial-flat) *path* as is.
+    This requires *path* to resolve to a regular file rather than directory.
     The result is identical to that produced by the GNU commands
     `md5sum` and `sha1sum`.