Update CONTRIBUTING.md

I've updated the README.md and the CONTRIBUTING.md to match the new requirements (Drupal >= 11.1.2) and to include better information about the ddev-drupal-xb-dev ddev addon.

What I don't know is if we should include sections like:

1. Prerequisites: to include it requires php8.3, Drupal 11.1.2, ...
2. Code structure overview: To explain how's the code structure (tests, tests modules, ui, ...)
3. Contribution workflow: I assume this is implicit in any Drupal contribution, but we can add a specific section about the usage of phpunit, phpstan, ... It's, more or less, added to both sections, but maybe something clearer.
4. Development best practices: Besides of the "use dependency injection,..." type of things, practices or contracts made in this specific project. Maybe like ADRs but not that strict.
5. Links to other projects related with XB?

Any other thing to add things to add?

Checked with the team working full-time on XB, and we're 50/50 on ddev vs "bare metal" on macOS.

Given that, we can be confident enough about recommending ddev. That was my key reservation.

Thanks for making these docs more welcoming, @isholgueras! 🙏🥳