Add documentation for array runtime lowerings for the internal API docs

[JinShil]

In my prior PRs (#2647, #2644, #2643, #2634), I did not realize we actually publish documentation on the internal API. This PR repairs that:

cc @Vild. I'm sorry, but I was wrong about not needing to document these; they need to be documented in the "Internal API" docs. You'll just need to add an entry to the mak/DOCS file for any new files.

I'd like to remove the runtime lowerings from the object.d documentation, but I haven't yet found a solution to that. I thought versioning it out with version(CoreDoc) would work, but it didn't because these templates are necessary for correct language behavior. I'm investigating other methods.

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Testing this PR locally

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[wilzbach]

I'm not sure what the problem here is? It's clearly marked as internal API, no?

Edit: I see you want to document them. Nevermind. Sorry.

[JinShil]

I'm not sure what the problem here is? It's clearly marked as internal API, no?

Yes, but it is publicly available at https://dlang.org/phobos/index.html

Currently it looks like this:

After this PR it will look like this:

So, this PR adds the new rt/array files, to maintain the status quo.

[JinShil]

Nothing in rt should be called directly by the user, but it is, nevertheless, currently documented and published at https://dlang.org/phobos/index.html

[wilzbach]

currently documented and published at

Yeah, it was an effort I made once to improve the documentation for developers.
Though most symbols don't have Ddoc comments, so while it's always welcome to improve the status quo / keep it, for now it's a failed effort.

[JinShil]

so while it's always welcome to improve the status quo / keep it, for now it's a failed effort.

We need to know which way to go though. If you'd like to maintain the status quo, please approve this PR. Otherwise, let me know what you'd like to do; should I submit an alternate PR to remove the rt folder from the docs?

[wilzbach]

I expect nothing. Just wanted to point that out.

[PetarKirov]

Of course it's not a big deal if we don't have documentation for all internal APIs, but we should strive have it regardless, as this helps the work of most contributors.