Allow references to useful subsets of large features

[ddbeck]

This is probably part of #91, but I wanted to call it out specifically so I can reference it from comments.

I want to be able to reference a subset of a larger feature, when it's useful to do so, without creating a new top-level feature.

Use cases:

• To durably reference features, even after they merge together. For example, someday we'll merge array-group into array. A Baseline banner in a blog post about array-group should still show the Baseline information for that feature, not the wider/older array feature.
• To reference historically distinct but now "invisible" features. For instance, I might want to provide a Baseline status for ES5 on caniuse, which (I think) are all part of the basic javascript feature now.
• To reference a specific part of a feature for more helpful discouraged feature alternatives. For instance, I might want to reference a group of JavaScript builtins from a discouraged feature (several examples in Add several discouraged JavaScript features #2670).
• To reference a logical group of keys for compute_from overrides or to pin the age of a feature when later keys are added. Perhaps we should have a conventional initial feature, to distinguish the keys that are "original" to that feature from those which were added later.

Spamming some phrases to make this easier to find later: inner features, inset features, interior features, merged features, minor features, historic features, subfeatures.

[ddbeck]

I've written some mock consumer documentation for feature aspects that builds on the work already in progress for redirects and splits. This is intended to set us up for merges.