docs(mpp): add Community services section with MPP Router

[shawnmuggle]

Adds a "Community services" section to the MPP on Stellar page, between "Examples" and "Learn more", with one entry: MPP Router.

• The section intro ("Third-party services built on MPP on Stellar.") is written so other projects can be added later, each under its own heading.
• Per CONTRIBUTING.md "Promotional and third-party content", the entry is descriptive: what the service is, how it uses MPP on Stellar (stellar.charge intent and payment channels), and which Stellar data and AI services it exposes. No pricing, fees, or comparative claims.
• The audit line links to the HackenProof report for the payment channel contracts.
• MPP Router is open source: https://github.com/mpprouter/rozo-mpprouter

Checked with pnpm format:mdx and pnpm check:mdx.

🤖 Generated with Claude Code