Document BlockControls component as preferred controls rendering

[aduth]

Related: #468, #830, #1019

This pull request seeks to update blocks documentation, removing references to controls and wp.blocks.registerControl, the latter of which was never implemented. In their place is new documentation for the BlockControls component, which can be returned in the edit rendered result to populate a block's toolbars. See #830 for real-world example usage.

View documentation

Open questions:

How should we approach ES2015+/JSX vs. ES5 in documentation? Thus far the majority of documentation has been written as ES5 style, with assumption that ES2015 will not be a prerequisite for registering blocks. An exception to this is that an external link is included to an equivalent ES2015+/JSX implementation of the example block, but this has been difficult to maintain especially as changes have been made to the original code snippet. GitHub Markdown files support the <details> element, which could be useful for collapsible equivalent implementations. As we move toward integrating documentation into the WordPress developer site, it might be nice to consider tabbable previews like PouchDB Callbacks/Promises/Async functions demonstrations or Stripe language demonstrations.

Testing instructions:

There are only documentation changes included in this pull request.

[youknowriad]

How should we approach ES2015+/JSX vs. ES5 in documentation?

My approach here would be to use ESnext. ESnext is now pretty commonly used in the JavaScript world. Maybe not in the WordPress world yet but we should make sure it gets more ❤️
I also think we should JSX to match the code.

But regardless this PR is 👍

[aduth]

For consistency's sake, I'm going to keep this as-is with the ES5 syntax, then we can revisit in a separate pull request which can be more targeted at the conversion (and hopefully invite more discussion in so doing).