OAS 3.2: querystring

Explain what querystring is about, why its different from query, and how it works.

Adapt `understanding-structure/parameter-serialization.md` and make it all make sense in a 3.2 world.

Start with understanding-structure/parameter-serialization.md and review how it currently describes query and parameter serialization. Clarify what querystring means, how it differs from query, and how each works in an OpenAPI 3.2 context. Done means the page is coherent and accurate for the 3.2 model.

Written by the indexing model from the issue text.