docs: add Modelflare custom endpoint example

[magnus-minor]

Adds an AI Endpoints page for Modelflare, an OpenAI-compatible AI API gateway, and lists it in the sidebar meta.json (alphabetical).

The page follows the structure of the OpenRouter page (API key, librechat.yaml config, notes). Base URL (https://modelflare.dev/v1, with https://origin.modelflare.dev/v1 as the mainland-China host), the /v1/models listing used by fetch: true, and the example model ID come from the Modelflare docs. English only; translations are left to the translation workflow. Prettier check passes on the changed files.

[vercel]

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

A member of the Team first needs to authorize it.