Add documentation for entity declaration design work

[jonmeow]

Trying to pull in key elements of #3762, #3763, and #3980 (decl matching and extern, essentially). These aren't specific to any particular declaration type, but are common to entities, so suggesting a new doc oriented on that.

There's probably more that could be said here, I'm just focused on getting the recent formal discussion mirrored into the design.

[josh11b]

It seems like this might be alternatively be used to improve build scalability by reducing dependencies. This is independent of breaking dependency cycles, and isn't specifically about build parallelization.

Suggested change

	- This may be used to break dependency cycles between libraries, which
	could be used to improve build parallelization.
	- This may be used to break dependency cycles between libraries, which
	could be used to improve build parallelization.
	- This may be used to reduce transitive dependencies, improving build
	scalability.

[jonmeow]

How about this rephrasing... (to copy here)

    -   This can be used to improve build performance, such as by splitting out
        a declaration in order to reduce a library's dependencies.

Note, trying to step aside from specific ways it might improve build performance because I want these bullets to focus more on what the feature does, versus how it might be used.

[josh11b]

I'm fine with the new text, but don't you want something about that it can allow you to break cycles too?

[jonmeow]

To me, that sort of feels like details where if we really want to explain how to use the feature this way, we should have more examples of refactorings to do.

[jonmeow]

i.e., I'm trying to figure out the right line to draw between proposal advocating for adoption of the feature, design trying to explain what it is, and user guide explaining how to apply it in anger.

[josh11b]

I feel like the design shouldn't just say what it is, but also the why, and the purpose of the feature is definitely in scope.