Document the concept of ggsql's internal representation

[teunbrand]

I couldn't find this on the website but ggsql exists as three parts: the reader, the builder and the writer.
We should explain:

• That there are three parts
• What the reader does
• What the builder does
• What the writer does
• How the parts interact, i.e. builder talks to reader for queries.
• For each part, what modules are available (e.g. the backends and writers)