Create a PR template for website banners

While a banner pull request is not a complicated change, non-technical people do not create their own when they want something done.

To empower non-technical people to create their own banner pull requests, we can add a Pull request template / form that will generate a pull request that is ready for review.

See recent https://github.com/adoptium/adoptium.net/pull/741/changes as an example of the types of changes that the template should mimic.

Start by reviewing the changes in pull request #741 and the repository's existing pull request configuration. Create a pull request template or form that captures the information and changes needed for a website banner, then verify that it produces a pull request ready for review and mirrors the relevant parts of #741.

Written by the indexing model from the issue text.