Enhance README with website link and quick start guide

[a-holm]

• [x] Closes #11838
• [ ] Tests added / passed (N/A for documentation-only changes)
• [x] Passes pre-commit run --all-files

This PR enhances the README by:

• Adding a link to the dask.org website
• Including basic installation instructions for both conda and pip
• Adding a minimal code example to demonstrate basic usage
• Specifying supported Python versions (3.9-3.12) which is otherwise very difficult to find out.

These changes help new users quickly understand what Dask is and how to get started, while still keeping the README concise and directing users to the full documentation for more comprehensive information.

[github-actions[bot]]

Unit Test Results

See test report for an extended history of previous test failures. This is useful for diagnosing flaky tests.

      9 files  ±0        9 suites  ±0   3h 11m 43s ⏱️ - 1m 35s  18 064 tests ±0   16 847 ✅ ±0   1 217 💤 ±0  0 ❌ ±0  161 650 runs  ±0  149 532 ✅ +3  12 118 💤  - 3  0 ❌ ±0 

Results for commit 9f58ef04. ± Comparison against base commit 6da0e73f.

:recycle: This comment has been updated with latest results.

[a-holm]

@TomAugspurger added the suggested changes, but I very much disagree since I don't think any of those solutions are very beginner-friendly. But it makes sense that you don't want the readme to grow stale, although I don't think it is such an issue to update it. I should try a few pull requests on the docs maybe and see if maybe I can do small changes there you like. It is important that maintainers also find it easy to deal with, and in that case I guess links like this is easier.

[a-holm]

I assume the [Tests / test (ubuntu-latest, 3.12, pandas-nightly) (pull_request)](https://github.com/dask/dask/actions/runs/14011005501/job/39230789971?pr=11841)Failing after 26m fails due to pandas-nightly and not because of a new README file.