Document the loadable extension feature

[vrischmann]

We can now build loadable extension but there's no documentation, we should write some.

[ConnorRigby]

    exe.addIncludePath(.{ .path = b.pathJoin(&.{ sqlite.builder.build_root.path.?, "c" }) });

i did a bit of fumbling around to come up with this in my build.zig to get building the extension working