tpch: document the queries in more detail

[rjnn]

We use the TPC-H queries in many different Distributed SQL tests. It would be helpful to have a single documentation page listing the various queries, a short description in prose of what the query does, and DistSQL query plan URLs (with the associated cockroach hash that generated that query plan), and link all known issues that are encountered when running those queries (e.g. such as known memory usages due to large Hash Joins).

This will also be helpful once we start manually rewriting the currently unsupported correlated-subquery queries and we want to keep track of what we're doing to those queries.