docs: resolve markdown linter issues

- Configure MD024 to allow duplicate headings among siblings in .markdownlint.jsonc.
- Replace pseudo-heading in DEVELOPING.md with a GFM alert block (MD036).

Co-authored-by: greenc-FNAL <2372949+greenc-FNAL@users.noreply.github.com>

Author: google-labs-jules[bot]

.markdownlint.jsonc

@@ -3,6 +3,9 @@
   "MD012": true,
   "MD013": false,
   "MD022": true,
+  "MD024": {
+    "siblings_only": true
+  },
   "MD031": true,
   "MD032": true,
   "MD034": true,

DEVELOPING.md

@@ -2,7 +2,8 @@
 
 ## Contributions
 
-_[The contribution policy is currently under development.]_
+> [!NOTE]
+> The contribution policy is currently under development.
 
 Except in rare circumstances, changes to the `phlex` repository are proposed and considered through GitHub pull requests (PRs).