Apply pull_request_template.md when creating pull requests

[rejc2]

When creating a pull request in the GitHub UI, the PR description will be pre-filled with the contents of pull_request_template.md in the root (or in .github or docs), if present. (See this doc for details.)

With the change here, Sapling will also pre-fill new PR descriptions with the contents of this file.

[meta-codesync[bot]]

@facebook-github-bot has imported this pull request. If you are a Meta employee, you can view this in D88544172. (Because this pull request was imported automatically, there will not be any future comments.)

[facebook-github-bot]

@rejc2 has updated the pull request. You must reimport the pull request before landing.