Document how to fine-tune timeout configuration #434

Motivated by #411

The current documentation on timeout featured in the quickstart is a bit scarce: it only shows how to override all timeouts using a float value.

We should document:

• How to fine-tune connect, read and write timeouts using httpx.TimeoutConfig(connect=..., read=..., write=...).
• How to disable timeouts. (Related: Passing 'timeout=None' should always result in no timeout #433)

Examples should ideally show how to do this on the top-level API (httpx.(timeout=...)) and on a client (httpx.Client(timeout=...), client.get(timeout=...).

To avoid clutter in the quickstart guide, this information should land in a new section in the Advanced Usage section (e.g. "Timeout fine-tuning"), and we should link to it from the timeout quickstart.