docs: add example for mTLS / corporate proxy configuration

[JakeBx]

Enterprise users need to configure mTLS client certificates and custom CA
bundles when connecting through corporate API gateways or TLS inspection
proxies. The SDK already supports this via the http_client parameter,
but there's no example showing the pattern.

OpenAI recently launched an mTLS beta program, and LangChain users are
requesting mTLS support for Anthropic (langchain-ai/langchain#35843).
An example in examples/ would make this capability discoverable.

Happy to implement.

[nightcityblade]

Hi, I'd like to work on this. I'll submit a PR shortly.

[karpetrosyan]

We have already documented that users can pass the HTTP client to Anthropic's client. I think we shouldn't go deeper with the httpx examples. Users should consider reading about this in the HTTPX docs.