Documenting instructions for attachments.

Also some document hierarchy changes to try to make the next steps more prominent.
Add Optimizely hook to page edit button

Optimizely can track this button by ID, class name, or complex selector.
All of these change from time to time, however, so having a dedicated
attribute will be ideal. It is also self-documenting, making it clear to
readers that the attribute cannot change without coordination with
Optimizely test managers.
Merge pull request #2496 from stephaniehobson/docs-install-attachments

Documenting instructions for attachments.
Merge pull request #2493 from openjck/optimizely-edit-button-hook

Add Optimizely hook to page edit button
Rename some GA event categories, actions, labels

Luke decided on these new names as part of ongoing event cleanup work.
Merge pull request #2495 from openjck/rename-ga-events

Rename some GA event categories, actions, labels