Update ecosystem docs for new clients and libraries

[ignatov]

Summary

• add the official Java SDK to the docs and navigation
• expand the community libraries page with additional ACP libraries
• expand the clients page with newly found ACP apps and messaging clients

Testing

• npx prettier --check README.md docs/docs.json docs/get-started/clients.mdx docs/libraries/community.mdx docs/libraries/java.mdx

[bbartels]

@ignatov https://github.com/dotnet/acpdotnet doesn't seem to exist, was this a typo?