[skip ci] Update pull_request_template.md: checklist for documentation updated (#1735)

Author: climbfuji

.github/pull_request_template.md

@@ -2,11 +2,11 @@
 
 _Note. The title and description will be used as commit log title and message for squash merges._
 
-## Dependencies
+### Dependencies
 
 _If there are PRs that need to be merged before or along with this one, please add "- [ ] waiting on LINK_TO_PR" for each of them._
 
-## Issues addressed
+### Issues addressed
 
 _Add any issues here that this PR closes or is related to. Use "Resolves" or "Closes" to automatically close issues when the PR is merged. Using "Working towards" or "Related to" for other cases._
 
@@ -34,3 +34,6 @@ List all systems intentionally or unintentionally affected by this PR.
 - [ ] This PR addresses one issue/problem/enhancement or has a very good reason for not doing so.
 - [ ] These changes have been tested on the affected systems and applications.
 - [ ] All dependency PRs/issues have been resolved and this PR can be merged.
+- [ ] All necessary updates to the documentation on readthedocs are included in this PR
+    - For site config updates, check in particular `doc/source/PreConfiguredSites.rst` and `doc/source/MaintainersSection.rst`
+- [ ] All necessary updates to the spack-stack wiki will be made when this PR is merged