add links to API definitions in docs

https://www.reddit.com/r/androiddev/comments/8vzqpq/android_ndk_glib_and_glibc_support/e5g99pe/

https://developer.android.com/ndk/guides/stable_apis ought to link directly to the version scripts that define each library so it's easy to see what is actually available and when.

Open the Android NDK stable APIs guide at developer.android.com/ndk/guides/stable_apis and inspect how its library availability is presented. Locate the version scripts that define each library, then add direct links for the libraries in the guide; done means readers can reach the relevant script for every listed library.

Written by the indexing model from the issue text.