Include instructions for permalinking relevant files

Why is this change being made?

I've noticed comments in various issues and MRs which reference lines in files without providing a specific commit, which means that when the file is eventually changed, the discussion may no longer make sense.

This MR adds details to the handbook to explain how to add a permalink to a specific commit when referencing a file in an issue or MR.

See also this accompanying MR: Document permalink button for linking to files

Author Checklist

• Provided a concise title for the MR
• Added a description to this MR explaining the reasons for the proposed change, per say-why-not-just-what
• Assign this change to the correct DRI
  • If the change relates to any part of the project other than updates to content and/or data files (e.g. team.yml) please make sure to ping @gl-static-site-editor in a comment for a review and merge.
  • If the DRI for the page/s being updated isn't immediate clear, then assign it to your manager.
  • If your manager does not have merge rights, please ask someone to merge it AFTER it has been approved by your manager in #mr-buddies

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