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, Matthew, Cigaryno

Maniphest Tasks: T15870

Differential Revision: https://we.phorge.it/D25822