change API documentation from time.core to just time

[eteq]

This is a pretty trivial PR that probably only @taldcroft will care about.

This changes the documentation for the time subpackage to be an API for astropy.time instead of astropy.time.core. Right now, this doesn't change what classes/functions are documented because astropy/time/__init__.py just has from .core import * in it.

I think it's better to do it this way, though, because now if I put astropy.time in the docs (or someone else does via intersphinx) it will link to that page, instead of having no link (because it thinks it's documentiong astropy.time.core). It's also marginally clearer this way to users that they should do from astropy.time import foo instead of from astropy.time.core import foo.

Or is there some other reason you had for using core over the base package, @taldcroft?

[taldcroft]

@eteq - looks good. I never had any desire to involve core in the story, this is just the result of my general confusion about how exactly sphinx works. From my experience if I did something like :class:'~astropy.time.Time' then this would fail, I had to do :class:'~astropy.time.core.Time'. (I'm using forward apostrophe instead of backtick because I don't feel like looking up how to escape in markdown).

Upshot, my mastery of sphinx is really limited, so please fix anything I'm doing wrong. 😄

[eteq]

@taldcroft - ok, just wanted to make sure. I totally see where you're coming from, because even with this change you still do have to do :class:~astropy.time.core.Time. I also find that annoying, and have it on my (infinitely-increasing...) todo list to try to figure out a way to make e.g., `:class:`~astropy.time.Time just work.

But it turns out (because of esoteric details of sphinx' internals and how they interact with automodapi) that this change works fine even though the linking of individual objects doesn't. So I'll go ahead and merge it.