A better way to store extension data on the environment

Problem

The approach to storing extension data is explained only vaguely.

From the env-merge-info description and
the todo extension tutorial I infer that one has to add arbitrary attributes to the env object.

This doesn't look very clean. It risks breaking the object and one has to be careful that the data is already added (lots of hasattr() checks).

Proposal

• Add a dict env.extension_data and recommend extensions writing there.
• Can one initialize the extension data instead of checking in various places if it exists? Would env-before-read-docs the right place? I.e.

def setup(app):
    ...
    app.connect("env-before-read-docs", setup_data)

def setup_data(app, env, docnames):
    env.extension_data['todo_all_todos'] = []

Start by reading the env-merge-info description and the todo extension tutorial linked in the issue, then trace the env-before-read-docs setup_data entry point. Determine how extension data should be initialized and persisted without arbitrary environment attributes or repeated hasattr() checks. Done means the extension API and initialization guidance clearly support a shared extension_data dictionary.