Make the generated methods self-documenting: return shapes and the Session conventions

[arrufat]

The docs site now publishes the Python reference generated from this repository's main branch (https://lightpanda.io/docs/reference/python-api, built by the docs repo's python-reference workflow), and the hand-written reference page is being retired. Two pieces of information that page carried cannot be derived from the package today:

1. Return shapes. Every generated method in lightpanda/_methods.py is annotated -> Any, so the reference cannot say whether a call returns text, parsed JSON (dict/list), or bytes. The retired page had a Returns column for this. Options: have scripts/generate_methods.py emit narrower annotations where the tool schema makes the shape known, or state the return shape in the docstrings taken from the tool descriptions.

2. Session conventions. Keyword-only arguments, snake_case names derived from the browser's action names, the selector / backend_node_id pair where one is required and selector wins, Session.call as the escape hatch with the browser's own names, and ToolError on failure. This is currently a fixed paragraph in the docs generator; it belongs in the Session class docstring so it also shows up in IDE hover text and stays next to the code it describes.

[arrufat]

From the docs review (lightpanda-io/docs#95): two more things the generated reference page can only show once the package carries them.

3. Per-argument descriptions for actions. The browser's tool schemas already describe every property (for example click describes both selector and backendNodeId), but scripts/generate_methods.py only emits the tool-level description. Adding an Args section to each generated docstring, in a format pdoc converts (Google or reST), would surface them on the docs page and in IDEs.

4. Docstrings for the hand-written surface. Browser.__init__ only describes args; binary, env, timeout and verbose are undocumented, and Browser.new_session, Session.close, CDPServer.close and BiDiServer.close have no docstring at all.