docs: JS + Go SDKs are live, link MCP server, fix endpoint count

[OussemaFr]

• "Other languages": JavaScript/TypeScript and Go were listed as coming soon but both shipped. Now linked to npm socialapis-sdk and socialapis-go.
• Added the MCP server (@socialapis/mcp / hosted https://mcp.socialapis.io/mcp).
• PHP: "coming soon" → "not available yet — tell us if you need it" (no committed date).
• Coverage row: "45+ endpoints" → "50 endpoints" (current API surface).

Docs only. PyPI shows the README from the last release, so this reaches PyPI with the next version.