docs: SELF-HOSTING.md

[AnayDhawan]

The headline deliverable: a stranger in another city can stand up their own StudyMap. Document the
full bring-your-own-data path.

Task

• Add SELF-HOSTING.md: fork → set studymap.config.ts (your city center/zoom/bounds) → add your
  data/places/*.json → npm run dev → deploy (Vercel / static export).
• Link it prominently from the README.

Acceptance: a stranger can follow it to run StudyMap for a new city.
Depends on: config-driven region + data-source abstraction.