Docs don't make location of GhApi clear

[henryiii]

I could not figure out where GhApi came from just by reading the docs, I tried ghapi.GhApi, I tried dir(ghapi), and no sign of it. I clicked through a bit and didn't see it either. Finally found it by entering colab and seeing the ghapi.core import at the top that gets hidden. Two recommendations:

• Somewhere in the docs mention that ghapi.core is the home for GhApi.
• Use qualified references in the docs. import * makes docs really hard to follow (especially when it's then hidden! But even not hidden it is bad in docs). Since most operations are on the ghapi.core.GhApi object, it won't make the docs that much more verbose, and would be much easier to follow.

PS: It's also missing from the readme.

[hamelsmu]

Added this to the README/docs

from ghapi.all import GhApi

cc: @jph00