Add missing event-routing docs

[saraburns1]

Add to https://docs.openedx.org/en/latest/developers/references/internal_data_formats/tracking_logs/course_team_event_types.html#grading-related-events

1. edx-course-grade-passed-first-time
   https://github.com/openedx/event-routing-backends/blob/master/event_routing_backends/processors/tests/fixtures/current/edx.course.grade.passed.first_time.json

Add to https://docs.openedx.org/en/latest/developers/references/internal_data_formats/tracking_logs/student_event_types.html

1. edx-completion-block-completion-changed
   https://github.com/openedx/event-routing-backends/blob/master/event_routing_backends/processors/tests/fixtures/current/edx.completion.block_completion.changed.json

2. edx-course-grade-now-passed
   https://github.com/openedx/event-routing-backends/blob/master/event_routing_backends/processors/tests/fixtures/current/edx.course.grade.now_passed.json

3. edx-course-grade-now-failed
   https://github.com/openedx/event-routing-backends/blob/master/event_routing_backends/processors/tests/fixtures/current/edx.course.grade.now_failed.json

More info in openedx/event-routing-backends#339 and openedx/event-routing-backends#399

References openedx/event-routing-backends#490

[sarina]

Note that in https://github.com/openedx/event-routing-backends/blob/master/docs/event-mapping/Supported_events.rst I've added links to this issue where this missing documentation should be linked to. As part of addressing this ticket, the updated event links should be added to https://github.com/openedx/event-routing-backends/blob/master/docs/event-mapping/Supported_events.rst.