R1:0fe0b9f681d0

Documentation: add article about Javelin DOM stuff

Summary:
Finally we know something about:

- how to get an element by id ($)
- how to find some children by a parent (scry)
- how to find an exact child by a parent (find)
- how to find a parent by a child (findAbove)

Closes T15870

Test Plan:
You read this article and you find something interesting and
probably also accurate.

Reviewers: O1 Blessed Committers, aklapper

Reviewed By: O1 Blessed Committers, aklapper

Subscribers: aklapper, tobiaswiese…