docs: Split community projects into sections, and add Java client

[vorburger]

Motivation and Context

I found the current raw assorted list of community project a bit overwhelming and confusing at first.

So I gave some thought to it, and realized that at first I primarily wanted to see what there is to browse the registry - so that should probably go first? (The current magnifying glass icon, which was even missing on one of the links, wasn't obvious enough, for me.)

Also I was looking for a Java client, and realized there already Go and TS clients in this list - let's group those.

The Other section in the middle is for everything else (for now, it can always further evolve, later).

How Has This Been Tested?

N/A, as documentation only.

Breaking Changes

Nope, it's documentation only.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

[domdomegg]

Nice, generally looks good. Just one comment above and we can get this merged ;)