add links to API definitions in docs

[DanAlbert]

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.