Add quickstart docs with simplest workflow

[Iain-S]

This PR aims to document the simplest possible workflow:

• make tables, make generators, create tables & create data
• No vocab
• No stats
• No config file

Even though it is good to keep a quickstart simple, I do think that we should add at least two more examples, to show the use of the stats and vocab functions. I think it should be fine to do that in separate PRs, though.

[Iain-S]

You can see the docs here https://sqlsynthgen--72.org.readthedocs.build/en/72/

[myyong]

Do you want to keep the sphinx dependencies where they are or move them over the dev dependencies?

[Iain-S]

For whatever reason, it isn't possible to do the optional-extras trick for dependencies in the dev group so they have to live in the main one.