Add a default pull request template for the organisation

[PeterJCLaw]

Summary

This aims to make it easier to submit pull requests with structured information which aids reviewers. We already have a few repo-specific templates, so this generalises the pattern they've set out. Individual repos can (and are encouraged to consider) have their own templates, though for many the default will be sufficient.

Code review

Testing

Hard to validate this before it's merged.
Have confirmed (via copy/paste) the file name spelling matches that from a known working example.

Links

Docs:

• https://docs.github.com/en/communities/setting-up-your-project-for-healthy-contributions/creating-a-default-community-health-file
• https://docs.github.com/en/communities/using-templates-to-encourage-useful-issues-and-pull-requests/creating-a-pull-request-template-for-your-repository

Prior art:

• https://github.com/srobo/ansible/blob/main/.github/PULL_REQUEST_TEMPLATE.md?plain=1
• https://github.com/srobo/team-emails/blob/main/.github/pull_request_template.md?plain=1
• https://github.com/srobo/volunteer-emails/blob/main/.github/pull_request_template.md?plain=1