Add custom GitHub action and pull request template to enforce that documentation is updated

[dddlr]

Added a pull request template with a checklist that asks that documentation be updated for every PR.

This is enforced by a custom GitHub action that checks whether any tasks in the checklist are marked as incomplete. If any are incomplete, the GitHub action will give an error.

You can exempt parts the PR description from the incomplete task check by adding <!-- task-checklist-ignore-start --> and <!-- task-checklist-ignore-end --> around the sections where you want to disable the check.

---

Test it out here!

• [x] Item in the checklist
• [x] Another item in the checklist
• [x] A third item in the checklist

---

Alternative methods considered for finding incomplete tasks in the PR description:

• https://github.com/kentaro-m/task-completed-checker-action no longer works
• https://github.com/venkatsarvesh/pr-tasks-completed-action doesn't give any way to skip the check

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Latest commit: ac49a30008e0a5cb04709ff8dd6825c313d5bb7f

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