docs: fix a bug with offline docs generation

This is a rather annoying bug. It was revealed because CLI reference stopped
working for *offline* docs after the previous commit. Turns out, none of the
plugins we enabled for normal docs were turned on in `mkdocs-offline.yml`.

Thanks to @mondeja for figuring out what was going on.

In the future, we could try a less ugly fix, e.g. turning the `offline` plugin
on or off via an environment variable.

See also:
squidfunk/mkdocs-material#6749
mondeja/mkdocs-include-markdown-plugin#195 (comment)

mkdocs-offline.yml

@@ -4,6 +4,14 @@
 INHERIT: 'mkdocs.yml'
 plugins:
     - offline
+
+    # For now, *every plugin config* from `mkdocs.yml` has to be repeated here.
+    # https://github.com/mondeja/mkdocs-include-markdown-plugin/issues/195#issuecomment-1933085478
+    # https://github.com/mkdocs/mkdocs/issues/3216
+    - include-markdown # For the CLI reference
+    - search
+    - redirects:
+        redirect_maps:
 # Turns out the `offline` plugin forces the following
 # option no matter what, and therefore shouldn't be
 # used in the main config file.

mkdocs.yml

@@ -31,6 +31,9 @@ theme:
 extra:
   version:
     provider: mike
+
+# IMPORTANT: any changes to plugins have to be duplicated in
+# `mkdocs-offline.yml`. See that file for more details.
 plugins:
     - include-markdown # For the CLI reference
     - search