Improve getting started with find_api_page

This does three related things:

* Adds a bit more to "getting started" so that new users know to look in the subpage documentation for the getting started sections.
* References the `find_api_page` function in getting started so new users know they can use it to explore functionality from an interactive session
* changes `find_api_page` so that it is accessible from the top level of astropy (e.g. `astropy.find_api_page`).  If this is controversial, I can easily back-out that commit and instead reference the original location (`astropy.utils`).

This doesn't need to be in the RC, but I think it's a good idea to get this in before the actual release @astrofrog @embray