Configure Read the Docs and point the README and project URLs at the hosted documentation

[avgvi]

Hosts the documentation on Read the Docs instead of linking to the .rst sources on GitHub.

• .readthedocs.yaml: Ubuntu 24.04, Python 3.12, Sphinx with fail_on_warning, dependencies from docs/requirements.txt (Django 5.2 for DjangoCache, Sphinx below 10). docs/conf.py already puts the repository root on sys.path, so the package is not installed in the build.
• README links and the new Documentation project URL point at https://diskcache-community.readthedocs.io/en/latest/. latest rather than stable because the v5.7.0 tag predates the config file and cannot be built; stable becomes usable from the next tag.
• docs/development.rst describes the hosting; the changelog gets an Unreleased section.

Verified locally: sphinx -T -W --keep-going -b html succeeds, all six README anchors exist on the built tutorial page, and the rstcheck, doc8 and docs tox environments pass.