docgen: add support for classes and groups of things

[oandregal]

At the moment, classes aren't well documented. Only the JSDoc from the class declaration is taken into account, but we should also include method information, etc. Example: token-list.

A different example that could fall in the same category, could be groups of things: functions or literal objects that contain other functions. See api-fetch as an example.

[oandregal]

One approach could be using the @class / @memberof / @constructor tags. Without the use of @memberof we can extract a lot of method info from classes, though.

[aduth]

functions or literal objects that contain other functions. See api-fetch as an example.

Another example I encountered in #15176 is the plugins object from @wordpress/data, which I explained in the pull request as desiring to better document but having unclear direction or support.

[oandregal]

Related #14332

[oandregal]

Also related #15186 See https://github.com/WordPress/gutenberg/pull/15176/files#r278829802

[oandregal]

I'm triagging issues we don't plan to tackle. We can reopen if it regains interest.