Add Read the Docs site for mesa-examples and rewriting Readme.md as per mesa main repo style

### Summary
mesa-examples currently has no docs site — everything lives in one long README.md, and its example list is already out of sync with what's actually in examples/, gis/, and rl/. This issue tracks adding a Read the Docs site (Sphinx + MyST, matching mesa/mesa) that auto-generates category pages from each example's own README, and trimming the root README down to a short pointer + install instructions. Related: mesa/mesa#3272 (example metadata standard — this is complementary, not a replacement; can move to metadata-driven generation later).

Tasks to be completed:-
-Docs infrastructure
[ ] Add docs/conf.py (Sphinx + MyST config, auto-discovers example READMEs by category)
[ ] Add docs/index.md (landing page + toctree)
-Dependencies & config
[ ] Add a docs extra to pyproject.toml (sphinx, myst-parser, pydata-sphinx-theme, sphinx-copybutton)
[ ] Add .readthedocs.yaml at repo root (fail_on_warning: false for now)
README cleanup
[ ] Trim README.md — replace the full per-example catalog with a link to the docs site
[ ] Add the License Notice paragraph agreed in mesa/mesa#3272
[ ] Add the Read the Docs badge (after project exists — see below)
Local verification
[ ] pip install -e ".[docs]" + sphinx-build -b html docs docs/_build/html builds with 0 warnings
[ ] Run once with -W and note/triage warnings (pre-existing content issues, not setup bugs — see PR description for the list)
[ ] Spot-check rendered pages in a browser: headings nest correctly, images load, no example silently missing
Read the Docs registration (needs readthedocs.org admin access — ask maintainer to set it up)
[ ] Import mesa/mesa-examples as a project on readthedocs.org
[ ] Confirm project slug resolves to mesa-examples.readthedocs.io
[ ] Trigger first build, confirm it goes green
[ ] set up automation rules for versioned docs, mirroring mesa.readthedocs.io's stable / latest
PR
[ ] Open PR, link this issue and mesa/mesa#3272
[ ] Tag maintainer for review
[ ] Address review feedback
[ ] Merge, confirm RTD auto-builds on merge to main
References
Sibling docs setup: https://github.com/mesa/mesa/blob/main/.readthedocs.yml
Metadata standard discussion: https://github.com/mesa/mesa/discussions/3272
In-progress metadata PR: https://github.com/mesa/mesa-examples/pull/423

Start with the sibling mesa configuration at .readthedocs.yml and inspect README files under examples/, gis/, and rl/. Then review docs/conf.py, docs/index.md, pyproject.toml, .readthedocs.yaml, and README.md as the planned change points. Verify the documented sphinx-build command, zero-warning build, rendered pages, complete example coverage, and the Read the Docs project setup.

Written by the indexing model from the issue text.