docs: document new rate limit on exportPersonalData endpoint (#12519)

PR #12519 (merged 2026-07-08) added a rate limit to the Graph `POST /graph/v1.0/users/{userID}/exportPersonalData` endpoint: 5 requests per minute, keyed per user (via endpoint-path keying, since the userID is part of the path). Requests over the limit now receive `429 Too Many Requests`.

This is a user/admin-visible API behavior change and should be reflected in:
- API/Graph endpoint documentation for `exportPersonalData` (mention the 5 req/min limit and the `429` response).
- Any admin-facing docs listing rate limits enforced by oCIS services, if such a page exists.

No new env var/config option was introduced (the limit is currently hardcoded: `exportPersonalDataLimit = 5`, `exportPersonalDataWindow = time.Minute` in `services/graph/pkg/service/v0/service.go`) — worth noting in the docs that it isn't currently operator-configurable.

Opened automatically per the PR-merged documentation-completeness check.