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

[ElliotFriend]

Hi, @shawnmuggle! thanks so much for the PR, and thanks for running a Stellar-MPP service!

I'd like to avoid creating listings of community services that just so happen to use various pieces of Stellar functionality. The other listings of the community services and/or providers in the docs are geared towards people building using the technology, which isn't quite the same thing this PR accomplishes.

I think it could fit in under the Examples section on this page quite well, though. Seeing an open source application running MPP on Mainnet is something builders could absolutely benefit from. Placing a link to the repo as an example would be very welcome!

Thanks!