docs(rest-api): proxy passthrough guide (psxdata-api 0.4.0)

[mtauha]

Documents the X-PSX-Proxy header added in psxdata-api 0.4.0 (psxdata/psxdata-api#6, released as v0.4.0).

• New guide, rest-api/guides/proxy.mdx (added to the REST API Guides nav). It covers:
  • what the header does, and that the server must opt in (a 400 explains this when it's off);
  • cache first: hits never contact the proxy, and misses go through it and are cached for everyone;
  • supported schemes and ports, and why https:// proxies are refused;
  • checks on every request vs. only on a cache miss: public addresses only, IP pinning, the 5-second reachability check, 10/min per IP and 4 concurrent;
  • an error table (400, 429, 502 proxy_unreachable, 503);
  • Python and JavaScript examples, plus a CORS and credentials note.
• rest-api/introduction.mdx: a short Proxy passthrough section linking to the guide, and error-table rows for 502 proxy_unreachable and the proxy 429.

The endpoint reference already shows the header, because the OpenAPI spec synced on the v0.4.0 tag.

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