fix(docs): pin the site section title so it renders as nixfiles

With no entry in the docs-site nav (removed there so awesome-pages can
discover the synced trees), MkDocs derives the section name from the directory
and title-cases it, rendering "Nixfiles". The previous hardcoded nav spelled it
lowercase. Setting title in docs/.pages restores that without reintroducing a
nav entry.

Verified by rebuilding the aggregated site locally with both source trees
synced as the workflow does.

docs/.pages

@@ -1,4 +1,10 @@
-# Section ordering for the MkDocs awesome-pages plugin on docs.lyrapup.pet.
+# Section title and ordering for the MkDocs awesome-pages plugin on
+# docs.lyrapup.pet.
+#
+# The title is set explicitly: with no entry in the site's nav, MkDocs derives
+# the section name from the directory and renders it title-cased as "Nixfiles".
+title: nixfiles
+
 # `index.md` is this repository's root README, copied in by the docs-site build
 # before this directory is synced over the top. The trailing `...` picks up any
 # page added later, so a new file needs no edit here.