Add Plugin Documentation

You are creating documentation for a new official GramIO plugin.

Arguments

The user provides [plugin-name] — e.g., media-cache or session.

Steps

1. Read an existing plugin page for reference — e.g., docs/plugins/official/session.md — to match the exact format.

2. Read AGENTS.md for conventions.

3. Create the EN plugin page at docs/plugins/official/{plugin-name}.md:

   ---
   title: {Plugin Name} Plugin for GramIO