Add RubyLLM::Tribunal to ecosystem page

[Florian95]

Description

This PR adds RubyLLM::Tribunal to the ecosystem page as requested in #570.

RubyLLM::Tribunal is an LLM evaluation framework that complements RubyLLM by providing tools for testing and validating LLM outputs.

Changes

• Updated page description to mention "LLM evaluation"
• Added RubyLLM::Tribunal to the introduction list
• Added new section for RubyLLM::Tribunal following existing format (after RubyLLM::Schema, before Community Projects)

Related

Closes the request from #570

[crmne]

Thanks for the RubyLLM::Tribunal addition. Content looks good to me.

If possible, could you enable "Allow edits by maintainers" on this PR? I’d like to do a quick wording/style pass in-place so it matches the rest of the ecosystem page before merge.

If not, no blocker from my side; I can merge this as-is and follow up with a docs polish PR right after.

[Florian95]

@crmne its good changes are made, you can merge it if you want