Mirror the Python adapter docs to the webdriver.io devtools site #295

Part of #277

CONTRIBUTING requires user-facing changes to update the README and be
mirrored to website/docs/devtools in the webdriverio repo in the same change.
The new devtools-backend CLI and the whole Python adapter are user-facing and
documented in-repo only.

Acceptance criteria

A devtools page for the Python adapter on the site, plus
the backend CLI flags, linked from the existing Selenium devtools page.

Size: S. Depends on #279.