doc: add format-md step to release guide

PR-URL: #41809
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Richard Lau <rlau@redhat.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Mestery <mestery@protonmail.com>
Reviewed-By: Darshan Sen <raisinten@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

doc/contributing/releases.md

@@ -312,6 +312,9 @@ of the list. The most recent release for each release line is shown in **bold**
 in the index. When updating the index, please make sure to update the display
 accordingly by removing the bold styling from the previous release.
 
+Run `make format-md` to ensure the `CHANGELOG_Vx.md` and `CHANGELOG.md` files
+are formatted correctly.
+
 #### Step 3: Update any REPLACEME and DEP00XX tags in the docs
 
 If this release includes new APIs then it is necessary to document that they