doc: add link fragments to CONTRIBUTING.md (#2507)

Added GitHub style link fragments to the `[Commit Squashing]`
sections of CONTRIBUTING.md

Fixes: #2506

Author: karlpvoss

CONTRIBUTING.md

@@ -250,7 +250,7 @@ That said, if you have a number of commits that are "checkpoints" and don't
 represent a single logical change, please squash those together.
 
 Note that multiple commits often get squashed when they are landed (see the
-notes about [commit squashing]).
+notes about [commit squashing](#commit-squashing)).
 
 #### Commit message guidelines
 
@@ -321,7 +321,7 @@ in order to evaluate whether the changes are correct and necessary.
 Keep an eye out for comments from code owners to provide guidance on conflicting
 feedback.
 
-**Once the PR is open, do not rebase the commits**. See [Commit Squashing] for
+**Once the PR is open, do not rebase the commits**. See [Commit Squashing](#commit-squashing) for
 more details.
 
 ### Commit Squashing