docs: add mdma.software website links + MCP.so venue

[mateusz-mr]

What does this PR do?

Adds the official project website (https://mdma.software) as a link across the repo, so the README and the MCP server metadata point to the product site — not only the GitHub Pages URL. Also documents the MCP.so distribution venue.

• README.md — adds a 🌐 Website link (mdma.software) as the first item in the header nav (Live Demo / Docs stay on GitHub Pages, where the full docs live).
• packages/mcp/server.json — adds websiteUrl: https://mdma.software (reaches the MCP Registry on the next mcp-publisher publish).
• packages/mcp/manifest.json — sets homepage to https://mdma.software.
• packages/mcp/README.md — adds MCP.so to the Distribution venues table + a release-checklist reminder.

No code or schema changes — docs/metadata only.

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing behavior to change)
• Refactor (no functional change)
• Documentation
• CI / tooling

Packages Affected

• @mobile-reality/mdma-spec
• @mobile-reality/mdma-parser
• @mobile-reality/mdma-runtime
• @mobile-reality/mdma-attachables-core
• @mobile-reality/mdma-renderer-react
• @mobile-reality/mdma-prompt-pack
• Blueprint: ___________

Metadata/docs for @mobile-reality/mdma-mcp only — no published package source changed.

Checklist

• I have read the CONTRIBUTING guide.
• My code follows the existing code style (pnpm format and pnpm lint pass).
• I have added or updated tests that cover my changes. (N/A — docs/metadata only)
• All tests pass (pnpm test). (no code changed)
• Type-checking passes (pnpm typecheck). (no code changed)
• I have added a changeset (pnpm changeset) if this change affects published packages. (N/A — no published package source affected)
• New or changed MDMA schemas are backwards-compatible (or marked as breaking). (N/A)
• Sensitive fields are marked with sensitive: true where appropriate. (N/A)

How to Test

1. Open README.md — the header nav now shows 🌐 Website linking to https://mdma.software.
2. Open packages/mcp/server.json — websiteUrl is https://mdma.software.
3. Open packages/mcp/manifest.json — homepage is https://mdma.software.
4. packages/mcp/README.md Distribution venues table includes an MCP.so row.