Contributor proxy SSL example fails with the documented API

[MengqiLi0000]

The development proxy setup in .github/CONTRIBUTING.md currently calls httpx.SSLContext(), which raises AttributeError. It also passes ssl_context= to httpx.Client, which raises TypeError.

The version in docs/contributing.md uses the correct ssl.create_default_context(...)/verify= approach, but omits imports and does not close the client.

I prepared a small documentation correction that uses a complete context-managed example in both guides and updates the obsolete proxy and SSL links in the GitHub guide. Both corrected code blocks were executed against a local CONNECT proxy and a TLS endpoint using a temporary CA certificate; both returned HTTP 200 with certificate verification enabled.

Would a PR for this documentation correction be welcome?

Prepared with OpenAI Codex assistance.

[0xamlab]

On current master (b5addb6, Python 3.15), both failures reproduce: httpx.SSLContext() raises AttributeError, and Client(ssl_context=...) raises TypeError. The stale calls are at .github/CONTRIBUTING.md:214-217. I also ran the ssl.create_default_context(...) / verify= form through a local CONNECT proxy to a TLS server with a temporary CA: one CONNECT, one TLS GET, HTTP 200. A focused PR correcting that block, plus imports and context management in docs/contributing.md:213-216, would address a verified documentation defect.