feat: document missing e2e group key management endpoints

While reviewing the inconsistencies noted in #171, I identified five e2e
group key management endpoints that exist as REST routes in the Rocket.Chat
source (apps/meteor/app/api/server/v1/e2e.ts) but are not documented in
the OpenAPI spec:

• POST /api/v1/e2e.acceptSuggestedGroupKey
• GET /api/v1/e2e.fetchUsersWaitingForGroupKey
• POST /api/v1/e2e.provideUsersSuggestedGroupKeys
• POST /api/v1/e2e.rejectSuggestedGroupKey
• POST /api/v1/e2e.resetRoomKey

These are all part of the E2E group key rotation flow introduced alongside
the suggested key distribution system.

I have drafted spec entries for all five, following the same pattern as the
existing e2e.* entries in settings.yaml. Happy to open a PR if this is
a good direction.

Closes part of #171