Add Documentation for Embedding Julia (fix #3111)

This is a chapter for the manual that covers how to embed Julia in C/C++ (fixes #3111).

When writing this I added some convenience functions to julia.h (see #4997) that make usage of the C API simpler.

One important missing topic is memory management. I simply do not know how long a pointer to an Julia value is valid until the GC kicks in (or how to tell the GC that there is a reference on a value in C).