docs: add "Evaluate Your Agent" page

[cwing-nvidia]

Summary

• Adds new "Evaluate" nav section and "Evaluate Your Agent" page to the Fern docs (main version).
• Documents how users with their own Python agents can use NeMo Gym environments directly over HTTP — without wrapping their agent as a Gym server.
• Covers the resources server HTTP API contract, cookie propagation, verify request schema, and includes a full working MCQA example.

Related: #1305 (ng_collect_rollouts external agent URL support)

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[cmunley1]

Suggested change

	**Goal**: Learn how to evaluate your own Python agent using NeMo Gym environments — without rewriting it as a Gym server.
	**Goal**: Learn how to import and evaluate your own agent in NeMo Gym with minimal changes.