Documentation: add provider-base(7), describing the base functions
The base functions are the first tables of function pointers that
libcrypto and the provider pass to each other, thereby providing a
baseline with which they can communicate further with each other.
This also contains an example for a ficticious provider, providing an
implement of a fictitious algorithm for a fictitious operation.
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/9409)