Zig: add documentation for the zig build system

[andrewrk]

extracted from #367

• explain purpose, it's supposed to replace make/cmake
• example of building a zig executable
• example of building a C library
• go over all the API in std/build.zig and make sure it's all documented
• fix #1035

[andrewrk]

After reviewing #4402 I realized this is actually a rather difficult issue, and so removed the "contributor friendly" label. Sorry about that.

[matjam]

If we could document at least the bare minimum things people might want to be able to do, that would be cool.

• build an executable
• build a zig library
• build an executable that depends on a zig library somewhere else
• build an executable that depends on a header not in a standard path
• link to a library that is not in a standard path