[doc] describe extending testdriver with bidi (#49168)
Extend Testdriver extension tutorial with description of adding new methods matching WebDriver BiDi commands.
Addressing #47565.
Apparently, the current version is outdated, as it refers to the non-existing SetWindowRectProtocolPart. I updated the protocol part name to avoid confusion with the action name, but leave SetWindowRectProtocolPart out of scope for this PR.
BiDi events are out of scope as well.