Add PR code review guidelines to the CONTRIBUTING doc

[robyngit]

Document code guidelines for submitting a PR. These are aspects of changes that will be checked for during code review:

• Make sure to test your changes across all themes shipped with MetacatUI (like arctic, dataone, default, knb), and also the themes in the metacatui-themes repo (like drp and sctld).
• Check how your changes perform on various screen sizes.
• If you’re tweaking an existing model, collection, or view, please review where that component is used elsewhere in MetacatUI to ensure there’s no unintended impact on those functionalities (especially since our testing coverage is not comprehensive yet).
• Make sure changes don’t slow down MetacatUI even more.. especially if the PR involves big additions.
• Check for robust error handling and user-friendly error messages.
• If the PR adds or updates dependencies, are they are necessary or do they add bloat the project? Is it redundant with other dependencies? Can we use a more lightweight alternative? Try to choose dependencies that have a long history and big backing community to increase the chances of them being maintained long-term.
• If applicable, verify the functionality of your changes for both signed-in and non-signed-in users.
• In addition to the JSDocs, if your changes impact something in the docs/guides, update those. If it’s a big feature that requires configuration by those running MetacatUIs, add a new guide.
• Make sure security implications are considered and addressed.