docs: add general API overview

[ijelacicbot]

Summary

• Replace the TODO placeholder in docs/source/in_depth/overview.rst with a General API Overview.
• Cover high-level vs low-level APIs, core types, and the typical run / async_run flow.

Motivation

Closes #29

The In Depth overview page was added as a stub (Add placeholder for API overview.) and never written. The placeholder asked for:

• important classes (Client, Submission, …)
• important functions (create_submission, get_submission, run, …)
• the difference between the high-level and low-level APIs

Changes

• High-level helpers: run / execute / async_run / wait / get_client, plus the judge0.api batch helpers.
• Low-level Client methods that map to Judge0 HTTP routes.
• Explicit note that async_run is not an asyncio coroutine.
• Short examples for wait-later execution, test cases, and an explicit client.

Test Plan

• uv run sphinx-build -b html docs/source ... -W succeeds
• Overview HTML contains the four sections and has no leftover TODO
• Docs deploy runs only on master (not on PRs)

Notes for Reviewers

create_submissions / get_submissions are documented as judge0.api.* because they are not re-exported from judge0.__all__.