Clean up "generic" intro pod files.

Files like dh.pod, etc., mostly duplicated the API-specific pod files.
Removed the duplicated content; that often mean the whole file could
be removed.  Some of the content about internals got moved into README
files in the source tree.  Some content (e.g., err.pod) got moved
into other pod pages.

Annotate generic pages, remove dup NAME

Reviewed-by: Richard Levitte <levitte@openssl.org>