Improve documentation about events

[mondeja]

When you access to https://mdpo.readthedocs.io/en/master/cli.html#md2po--e the documentation of this option points "See the documentation for available event names." but without including a link because the events are now documented inside a parameter of the Python API.

It would be great to rename the page commands.rst of the manual to custom-extraction.rst and make "Commands" a subsection, adding another subsection explaining custom events.