docs: add MDMA to the Generative UI specs table

### What
The Generative UI specs table in [`docs/concepts/generative-ui-specs.mdx`](https://github.com/ag-ui-protocol/ag-ui/blob/main/docs/concepts/generative-ui-specs.mdx) currently lists A2UI, Open-JSON-UI, and MCP-UI. I'd like to add **MDMA** (Markdown Document with Mounted Applications) as another entry.

### Why
Every renderer handles JSONs but MDMA is a Markdown-based, LLM-friendly Generative UI format where interactive components (forms, tables, approval gates) are declared in fenced ` ```mdma ``` ` code blocks. It integrates with AG-UI via the community adapter **`@mobile-reality/mdma-agui`**, so it belongs in the same reference table alongside the other specs.

- MDMA repo: https://github.com/MobileReality/mdma

### Scope
A single appended row to the existing table — docs only, no code/integration changes. I'd frame the row as adapter-based (community), not first-party.

Happy to be assigned — I already have the one-line change ready to PR.