Add /doc/{flate,rustdoc} to .gitignore

[lilyball]

No description provided.

[alexcrichton]

Hm, this is getting to the point where we need a different way to go about this. We're going to add lots and lots of crates soon, and I don't want to keep doing this. I think we can have an ignore file like this:

doc/*
!doc/*.md
!doc/lib

Can you see if that'd work?

[lilyball]

@alexcrichton We could certainly introduce a doc pattern like that, I'm just mildly worried that this could cause problems if someone wants to introduce a different filetype into this dir, or even a subdir that's not generated by rustdoc. For example, git ls-files says that doc/po is a subdir that has tracked files in it, so that would need to be unignored as well.

[alexcrichton]

I'd rather not have to keep adding lines for every crate we add to the build process, so we have the option of using patterns and just remembering to un-ignore things, or we could also generate docs into the build directory instead of the doc directory (which perhaps we should be doing anyway).

[brson]

We should change the location where docs are built. This also causes us a problem with make clean.

[brson]

Not that we can't do this for now.

[brson]

I r+ed because I think we should do the fix in #11860 to squash this problem for good.