Update PR template

[cyschneck]

PR Summary

Closes #455

Changes to the PR template (.github/PULL_REQUEST_TEMPLATE.md):

• remove item about tagging the entire NCAR/geocat team
• update link to contributor's guide (depends on Contributor's Guide #450)
• add a note to squash and merge if appropriate for cleaning up commit history
• "remove link to non-existent "documentation standards" on geocat website" -> set to Docstrings description in contributions docstrings
• Reword the "add summary here" comment at top to indicate a written summary should be added in the section in addition to noting the corresponding issue

PR Checklist

General

• Make an issue if one doesn't already exist
• Link the issue this PR resolves by adding closes #XXX to the PR description where XXX is the number of the issue.
• Add a brief summary of changes to docs/release-notes.rst
• Add appropriate labels to this PR
• Make your changes in a forked repository rather than directly in this repo
• Open this PR as a draft if it is not ready for review
• Convert this PR from a draft to a full PR before requesting reviewers
• [n/a] Request @NCAR/geocat for reviews
• [n/a] Passes precommit. To set up on your local, run pre-commit install from the top level of the repository. To manually run pre-commits, use pre-commit run --all-files and re-add any changed files before committing again and pushing.

[anissa111]

Small point, but could we put the squash and merge item last in the general section?

[cyschneck]

Updated!