docs: Nano Banana 2.1 native API page and image billing

[liyangbing]

Summary

• New native GenerateContent page for google/gemini-nano-banana-2.1 (cURL, image edit, Google Gen AI SDK, billing), linked from the native API index, the GenerateContent guide and the sidebar.
• Add a Billing section to the Nano Banana Pro and Nano Banana 2 native pages: output image tokens are now billed at Google's image price (Pro $120/M, NB2 $60/M, NB2.1 $30/M), live in prod since 2026-10-08 (sandbase-registry#358 and the native billing fix, sandbase-monorepo#1065 / v2.92.112).

Tested

• npm run validate passes (OpenAPI lint, links, SEO, build, built public surface).
• The 2.1 cURL example was run against prod (short path + x-goog-api-key): 200, one JPEG, 1120 image tokens.
• Not tested: SDK snippet, streaming, search grounding (search is not documented).