docs: add BoostRail custom endpoint guide

[leegao6072-watershed]

Summary

• Add BoostRail to the AI Endpoints docs and sidebar, following the existing provider-page conventions.
• Document the endpoints.custom YAML, API key setup, fetch: true model loading, BYOK-only models in the fetched list, and the user_provided key option.
• Add a monochrome BoostRail provider icon (24×24 SVG, currentColor).

Verification

• pnpm typecheck (fumadocs-mdx + tsc) passed.
• pnpm test: 421 tests passed.
• Prettier and ESLint passed on the changed files.
• The YAML example validates against configSchema from librechat-data-provider 0.8.524 (config version 1.3.17).
• Live checks on 2026-10-07 against https://api.boostrail.com/v1 with a test key: GET /v1/models (the request LibreChat makes with fetch: true) returned the model list, and chat completions worked with and without streaming.

Disclosure: I work on BoostRail.

[vercel]

@leegao6072-watershed is attempting to deploy a commit to the LibreChat's projects Team on Vercel.

A member of the Team first needs to authorize it.