[docs] Add "Support" page

## Summary

This issue is about fixing a few problems at once using a new support page:

1. The instructions on how to get support as a user are unclear. https://base-ui.com/react/overview/about is the closest but doesn't really provide pointers.
2. There is no coverage of commercial support. However, we are about to release priority support on rich components which have/will have Base UI as a dependency so fixing problems there can also mean fixing problems here too.

## Solution space

We have a solution for this problem in:

- https://mui.com/material-ui/getting-started/support/
- https://mui.com/x/introduction/support/

the best option seems to copy this here.