[GitHub] Add AI Disclosure section to PR Template. (RestContract version)

[tdonohue]

Description

This PR is a direct port of DSpace/DSpace#13035 to the dspace-angular codebase. It adds our new AI Contribution Policy to the PR Template. See the other PR for more details. (The changes to CONTRIBUTING.md are NOT included because that file does not exist in RestContract, as it's more specific to code contributions)

NOTE: This PR includes a few other minor updates for the RestContract repository, as it has gotten "out of sync" with the DSpace/DSpace and DSpace/dspace-angular repositories

• First, the PR template did not exist at all. So, this PR creates the PR template (based on the one in DSpace/DSpace) and adds the AI disclosure section.
• Second, the issue template was very minimal/outdated. This makes minor updates to the issue template to align with the structure we use for DSpace/DSpace.

[tdonohue]

While this includes additional changes (beyond DSpace/DSpace#13035) to re-sync this RestContract repository, the additional changes are very minor/obvious. So, I'm merging this immediately. (If anyone finds issues in the changes made in this PR, we can always revert or update them in a future PR)