JSON API for group details and events

## Summary

After the migration from community.cncf.io to OCG, community group websites that were pulling data programmatically from the Bevy/Meetup API no longer have a way to fetch group details, events, or member counts.

## Use Case

Our CNCF chapter website ([cloudnativesdq.org](https://cloudnativesdq.org)) was scraping `https://community.cncf.io/cloud-native-santo-domingo/` to extract upcoming events, past events, organizers, and member count via the `__NEXT_DATA__` JSON blob. After the OCG migration, this data source is gone and there is no public API alternative.

Other CNCF chapters likely face the same issue if they maintain independent websites.

## What We Need

A public, read-only JSON endpoint to fetch:
- **Group details**: name, description, members count, organizers, social links
- **Upcoming events**: title, date/time, location, kind (virtual/in-person/hybrid), image URL
- **Past events**: same fields

Something like `/{community}/group/{group_slug}/json` (as proposed in #448) would be ideal.

## Current Workaround

Without a public API, community websites must resort to fragile HTML scraping of OCG pages (`ocgroups.dev/cncf/group/{slug}`), which will break whenever the frontend HTML structure changes.

## Context

This was previously raised in PR #448, which was closed with the note that an API is planned. This issue tracks the community need for that API.