[DOC release] Initial pass at filtering out api docs behind FFs

[toddjordan]

This PR is to address issue #13101

It uses a yuidoc preprocessor to parse features.json and determine features that are not yet turned on by default. If not turned on, any class with an @category tag matching that feature will be filtered out of the api docs class list.

[toddjordan]

cc @rwjblue
This PR is currently filtering out the 2 engines classes according to features.json. We will need to figure out other classes associated with features and label their @category accordingly.

[homu]

☔ The latest upstream changes (presumably #12211) made this pull request unmergeable. Please resolve the merge conflicts.

[rwjblue]

This looks great! I think we also need to be able to filter @method and @property declarations too (seems like this is working on @class only right?).

[toddjordan]

right. I'll work on a separate PR for that

[rwjblue]

Sounds good, we'll land this once Travis is happy (prior failure was unrelated and caused by Sauce Labs issue).

[toddjordan]

@rwjblue Travis happy now 😀

[rwjblue]

Thanks @toddjordan!