Fix the link to guidance on how to do Git commits

The linked Confluence pages give me a 404. Searching Confluence for "Git
commit" gives me [1], which suggests that the canonical source for this
data is actually the guidance that was added to this repo in ef8ac8f.

[1] https://ably.atlassian.net/wiki/x/eoChEg

best-practices/pull-requests.md

@@ -30,7 +30,7 @@ In order for a PR to be merged, everyone who has been tagged as a reviewer must
 
 ## PR scope and structure
 
-The scope and structure for a PR should be based on the guiding principles that apply to [commit and history structure](https://ably.atlassian.net/l/c/7QP1L31X): PRs should ideally cover a self-contained set of changes, with an overall scope that is large enough that the overall intent of the changes is clear, and small enough that it is manageable to be reviewed in detail. PRs that are too large tend to result in reviews that are more superficial and less effective.
+The scope and structure for a PR should be based on the guiding principles that apply to [commit and history structure](commits.md): PRs should ideally cover a self-contained set of changes, with an overall scope that is large enough that the overall intent of the changes is clear, and small enough that it is manageable to be reviewed in detail. PRs that are too large tend to result in reviews that are more superficial and less effective.
 
 The use of branches generally should follow the policy for [development flow](https://ably.atlassian.net/wiki/spaces/PUB/pages/803766520). If the changes being made are necessarily extensive, then the preferred way to structure the work is as a series of PRs made against an integration branch, so each individual PR remains a manageable size.
 
@@ -92,7 +92,7 @@ Changes that are made in response to PR reviews can include:
 The choice as to how to proceed principally rests with the author, and the actual choice should be made based on the goals of:
 
 - ensuring clarity for the reviewer(s);
-- ensuring that the final history meets the [goals for commit history](https://ably.atlassian.net/wiki/Git-best-practices).
+- ensuring that the final history meets the [goals for commit history](commits.md).
 
 Adding `!fixup` references allows the specific corrective changes to be seen clearly, and preserves the navigability of the original PR comments. However, depending on the nature and extent of the changes, `!fixup` commits can impair the reviewability of the PR overall - a reviewer may request that changes are consolidated into the actual proposed commits in this case. When this happens, a new branch and PR should be used referencing the old PR, explaining why a new PR is being used (what material change has been made) and the old PR should be closed.