OpenBLAS

Add documentation on redistributing OpenBLAS

This touches on the following:

- build configurations
- naming of symbols, shared/static libraries and other build outputs
  like pkg-config and CMake files
- (in more detail) guidance on ILP64 builds

It tries to explain that, while this is only guidance and there may be
reasons to deviate from that, for some build options there are best
practices, and for some others there are choices to make.

It also links to a number of well-maintained build recipes in order
to help packagers of other distros make choices.

Closes gh-3798

[skip ci]