Add documentation for `meta/tags`
Today I found the [`meta_tags` test](https://github.com/kaitai-io/kaitai_struct_tests/blob/dfdab556e4bb72096aa23b2146ba928928d8d783/formats/meta_tags.ksy) in the tests repo checking the availability of `meta/tags`. I had no idea that we have this in Kaitai Struct 🙂 I wondered what it's intended for, but I found no match in the documentation about it, just the issue https://github.com/kaitai-io/kaitai_struct/issues/572.

Reading the issue, it seems like a good idea, it would offer some capabilites to make the formats gallery more usable and organized. Hence I think the documentation should at least mention it.