Document all config expansions in a single page

[ssbarnea]

Issue

I want to have one documentation page that lists all expansions made by tox when loading its config, with the kind of values they would have. Example:

• basepython - full path to python interpreter
• env_name - name of the environment
• env_dir - path to the environment
• env_tmp_dir - {work_dir}/{env_name}/tmp
• passargs- ...

It should be noted that most of these do have some documentation, but they are not listed in a centralized space, making hard to determine what you could use.

I am curious if there is a programmatic way to generate these as I would also find it useful to list them with --help.

[gaborbernat]

You can refer to anything on https://tox.wiki/en/4.18.0/config.html really, so that page is the doc. We could add a const label to some, which are the injected values.