Update /docs pages to reflect current place categories and features (v2.2.x)

[AnayDhawan]

Summary

Several /docs pages have stale content that predates the v2.0.0–v2.2.1 refactors. This issue tracks updating all affected pages to reflect the current state of the map.

Pages to review and update

/docs/exam-centres

• Remove IB and IGCSE mentions (done in v2.2.1)
• Update filter label from "Exam Centres" to "SAT centre" / "Foreign lang exam centre"
• Update pin colour descriptions (purple = SAT, cyan = foreign lang)
• Add a note linking to the open issues for adding centres by country

/docs (index)

• Update the category list — sat_centre, foreign_lang_exam_centre, gov_offices, other_places are new; old categories (book_shop, exam_centre, repair_shop, etc.) should not appear
• Mention gov_offices as a category (passport offices, RTOs, post offices)

/docs/contributing

• Audit for any references to deprecated types (exam_centre, book_shop, repair_shop, stationery, internet_cafe, train_station, imp_locations)
• Add section on how to contribute SAT centres for a new country

General

• Verify all internal links still resolve (some may point to old routes or anchors)
• Check that the "valid place types" list in any docs page matches the 6 types in src/lib/types.ts

Why now

The v2.1.0 and v2.2.0 releases changed the type taxonomy significantly (8 types removed, 3 new ones added). Docs were partially updated in code but the /docs UI pages still have stale references.