New documentation

- [ ] API documentation
- We'll probably write all the API docs in the README.md
- #2615
- #2650
- #3904
- #3933
- #3959
- [ ] Examples / demos
- [x] Remove the old docs
- [x] #2653
- [x] #2648
- [x] #2655
- We need to review the examples we have, polish them, and see if they're relevant/pertinent
- [x] Update the `description` field in `package.json` #2644
- [x] https://github.com/adazzle/react-data-grid/community
- [x] Add one or more issue templates
- #2645
- #2649
- [ ] Add a pull request template?
- [ ] README.md
- [ ] Review badges
- [ ] Review links
- [x] Review installation guide #2646
- [x] Review quick start #2646
- [x] Features list #2642
- [x] List browser / react support #2642
- [ ] Performance Considerations
- [ ] CONTRIBUTING.md
- [ ] How to develop
- [ ] How to test
- [ ] How to contribute
- [ ] Write change logs up to a stable 7.0.0 release

Start by reviewing the remaining unchecked items in README.md and CONTRIBUTING.md, including API documentation, examples, badges, links, performance considerations, and development/testing guidance. Check the referenced issues and the merged pull request #2615 for context. Done means the applicable documentation checklist items are completed and the README and contributor guidance are updated.

Written by the indexing model from the issue text.