docs: document full StatboticsClient API surface and retry behavior

[cappy-dev]

On behalf of @Project516

Summary

The README only documented two of the five public methods on StatboticsClient. This expands the docs so the full API surface is discoverable without reading the source:

• Added an Installation section (pub.dev package + Git fallback), matching the style used in tba_client and firestore_client.
• Added an API reference section listing all five methods in a table with their REST endpoints and return types:
  • getEvent (was missing)
  • getEvents
  • getEventTeams
  • getEventTeamsBasic (was missing)
  • getEventMatches (was missing)
• Documented the per-method sort order (rank, week/name, comp level then match number) and the 404 -> empty-list / null behavior.
• Added a Retries and backoff section covering the 429/5xx retry with exponential backoff, the maxAttempts default, the sleep seam for tests, and that 4xx is not retried.
• Mentioned the close() lifecycle method in the quick-start example and the API reference intro.

Verification

• README-only change; no source or test code touched.
• Verified no em dashes or non-ASCII characters were introduced.
• API details cross-checked against lib/src/statbotics_client.dart and the existing tests in test/statbotics_client_test.dart (retry behavior matches the retries a transient 500 then succeeds (#496) test, and sort orders match the implementations).

[cappy-dev]

cc @Project516