doc: _extensions: Use github form to report documentation errors

This change makes the "Report an issue" button in the documentation
actually pre-fill the bug template as opposed to just opening a "plain"
issue.

The required "Describe the bug" field is not pre-filled at all, making it
mandatory for folks to put in effort into their report where before they
would have been able to just hit "Submit" :)

Fixes zephyrproject-rtos/zephyr#93654

Signed-off-by: Benjamin Cabé <benjamin@zephyrproject.org>