formats.kaitai.io — add some pointers to runtime installation
Following up https://github.com/kaitai-io/kaitai_struct/issues/565, it looks like it's not very clear for newcomers, landing at formats gallery website, that KS typically requires installation of runtime library for the proposed precompiled formats to work.

We should try to add some pointers on how to do that, but the catch is that we don't want to generate many pages of documentation, as this will impede visibility of generated source.