docs: add ReadTheDocs MkDocs scaffold

[raoabinav]

Evaluator Summary

• Abi added the hosted documentation scaffold for LEANN using MkDocs and ReadTheDocs, including navigation, landing pages, and strict docs-build CI.
• The design keeps docs as repository-native Markdown with a strict build gate, so broken links or stale docs fail before publication.
• The implementation adds ReadTheDocs config, MkDocs config, docs workflow, contributor docs-build guidance, and fixes links that become enforced by the stricter build.
• Quality bar: strict MkDocs build and diff checks were run; this PR intentionally sets up hosting infrastructure but does not claim the public site is already published.