docs: add Environments, Benchmarks & Data Generation guide

Comprehensive developer guide covering:
- Architecture (BaseEnv → HermesAgentBaseEnv → concrete envs)
- All three benchmarks (TerminalBench2, TBLite, YC-Bench)
- Training environments (TerminalTestEnv, HermesSweEnv)
- Core components (AgentLoop, ToolContext, Tool Call Parsers)
- Two-phase operation (Phase 1 OpenAI, Phase 2 VLLM)
- Running environments (evaluate, process, serve modes)
- Creating new environments (training + eval-only)
- Configuration reference and prerequisites

Also updates environments/README.md directory tree to include
TBLite and YC-Bench benchmarks.

website/sidebars.ts

@@ -64,6 +64,7 @@ const sidebars: SidebarsConfig = {
       label: 'Developer Guide',
       items: [
         'developer-guide/architecture',
+        'developer-guide/environments',
         'developer-guide/adding-tools',
         'developer-guide/creating-skills',
         'developer-guide/contributing',