Add `libparsec_testbed` crate docs #21722

Job	Run time
📊 Q&A	1m 10s
dispatch	3s
spelling / cspell	18s
docs	0s
rust / 🐧 Linux: 🦀 Rust tests	6m 46s
rust / 🍎 macOS: 🦀 Rust tests	1m 45s
python / (🐧 Linux only): 🐍 Python server tests	3m 24s
rust / 🏁 Windows: 🦀 Rust tests	14m 2s
web / 🌐 Web tests	12m 21s
⭐ CI is happy ⭐	1s
	39m 50s

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