docs: don't compute rl-next.md during build

not sure why this was done the way it was considering that includes are
a feature the doc toolchain had previously. let's just always have some
kind of entry for the upcoming release in the dev manual builds even if
that means having a completely empty release notes chapter.

the release notes generation script isn't entirely functional right now
due to pre-commit hooks, but it's good enough for time being. we need a
better release process for notes anyway.

Change-Id: Ifda6912cf5233db013f72a30247a62d6f22b1565

Change-Id: I9eb347ec4aabc5be2b816ff0fd3e4be45f93b934

doc/manual/src/SUMMARY.md.in

@@ -109,7 +109,7 @@
   - [CLI guideline](contributing/cli-guideline.md)
   - [C++ style guide](contributing/cxx.md)
 - [Release Notes](release-notes/release-notes.md)
-{{#include ./SUMMARY-rl-next.md}}
+  - [Upcoming release](release-notes/rl-next.md)
   - [Release 2.18 (2023-09-20)](release-notes/rl-2.18.md)
   - [Release 2.17 (2023-07-24)](release-notes/rl-2.17.md)
   - [Release 2.16 (2023-05-31)](release-notes/rl-2.16.md)

doc/manual/src/release-notes/rl-next.md

@@ -0,0 +1,3 @@
+# Upcoming release
+
+{{#include rl-next-generated.md}}