doc/release-notes: link the upcoming release notes again

The insertion marker comment broke the list into two parts, the first
containing only the link to the upcoming release notes and the second the
past releases. This confused the generator, leading to the first part being
discarded. Indent the marker comment so that it's syntactically part of the
preceding item, and in particular doesn't split the list any more.

Change-Id: I357c51bb03e4e0d79a76d30158615fd9eda95ea8

doc/manual/src/SUMMARY.md

@@ -196,7 +196,7 @@
   - [C++ style guide](contributing/cxx.md)
 - [Release Notes](release-notes/release-notes.md)
   - [Upcoming release](release-notes/rl-next.md)
-  <!-- RELENG-AUTO-INSERTION-MARKER (see releng/release_notes.py) -->
+    <!-- RELENG-AUTO-INSERTION-MARKER (see releng/release_notes.py) -->
   - [Lix 2.90 (2024-07-10)](release-notes/rl-2.90.md)
   - [Nix 2.18 (2023-09-20)](release-notes/rl-2.18.md)
   - [Nix 2.17 (2023-07-24)](release-notes/rl-2.17.md)

releng/release_notes.py

@@ -16,7 +16,7 @@ def add_to_summary(date: str):
     if VERSION_RL.exists():
         return
 
-    MARKER = '<!-- RELENG-AUTO-INSERTION-MARKER'
+    MARKER = '  <!-- RELENG-AUTO-INSERTION-MARKER'
 
     new_lines = []
     for line in SUMMARY.read_text().splitlines():