docs: guide, environment, protocol and contributing pages match the code

[tactino]

An audit of the user guide, environment, protocol and contributing pages against the code found commands that fail as written, and contracts that leave out what an env has to do. This fixes those pages, in both languages.

Commands that failed

• The dummy connectivity check failed on the first step. It now runs as written: plugrl-server #107 matches the dummy policy to dummy-v1, and every client command passes --server-host 127.0.0.1, since the default 0.0.0.0 cannot be connected to on Windows.
• The plugrl-run-* commands after uv sync were not on PATH. Every page now runs them with uv run, from inside the repository.
• The custom env example could not run. It set no action space, returned scalars where arrays belong, and never said where the file goes. It is rewritten as a small point env, and the page covers both placements: in-tree *_env.py, or a script ending in cli.main().
• LIBERO's flag is --runner.use-env-lock, not --use-env-lock.
• The environment list is now one row per env ID with its extra. It had the IDs wrong and implied most need no extra.

Contracts that were missing

• Custom env:
  • best_reward_threshold_for_success, without which rollout/success is always 0;
  • reset_indices;
  • seeding through seed_rngs/np_random;
  • no auto-reset;
  • config defaults and JSON-serialisable kwargs.
• Protocol:
  • the time-major action layout;
  • the terminal observation;
  • what the servers read from info (only episode, as SPEC.md now says);
  • what the conformance checker does and does not check (the page overstated it).
• Output and runs: where output goes, --resume and --exp-name, recording, and the robomimic variants.

What was run. The dummy pair and the custom env were run end to end against a real server on this laptop. The custom env was run both out of tree and in tree, with 2 envs and with 2 processes. Every flag was checked against the real --help. The LIBERO and robomimic commands were checked against their config classes, since those extras are not installed here. mkdocs build --strict passes.

Merge after these, because the pages describe their behaviour:

• plugrl-server #107 (dummy defaults) and #108 (a malformed info closes one connection, not the server);
• plugrl-env-client docs: the quickstart on the home page could not be run #11 (LIBERO installs MuJoCo 2.3.7), docs: the environment side needs neither CUDA nor a GPU #12 (the video extra) and docs: checking a client is one command now #13 (the state-only robomimic variants).