[Meta] Improve documentation

Problem/Motivation

Documentation now lives in our repo, which is exposed at https://docs.memberplatform.org/. Currently this is dummy content. This is a meta issue to track child issues to improve the documentation there.

Remaining tasks

• #3515927: Finish writing README.md
• #3515994: Add contributing file (CONTRIBUTING.md)
• #3515995: Add changelog file (CHANGELOG.md)
• #3516155: Use the new logo in documentation