Add PR template and update contributor docs

[edwardchalstrey1]

Issues closed by this PR

• Closes [Documentation]: Pull request and branching team practices #651

How to review this PR

See the comment below which includes how the PR template will look by default and review the contributor page at https://gambitproject--691.org.readthedocs.build/en/691/developer.contributing.html#contributing-code

[edwardchalstrey1]

Thanks for contributing to Gambit! Before you submitting or reviewing a pull request, check out our guidelines for contributing.

The person submitting the PR should ensure it has an informative title and update the headers below, before marking the PR as ready for review and assigning reviewers.

Issues closed by this PR

Add any issues that are being closed by this PR to the list. Use "Closes" or another keyword followed by the issue number.

• Closes #ISSUE_NUMBER

Description of the changes in this PR

Update the description of the changes made in this PR. You can delete this section if the changes are already fully described in the linked issues.

This PR ...

How to review this PR

The gambit repository contains source code and documentation for several different components. Explain how a reviewer should approach reviewing the specific changes made in this PR.

For example, you might include instructions like:

• "Review code changes and ensure tests cover edge cases"
• "Rebuild the GUI from this branch, then test features X, Y, Z work as expected"
• "Click the link to the documentation page and sense check by reading"