expand documentation and add more to `\?` command

[kenshaw]

Current usql documentation needs to be expanded to provide the same level of documentation level as psql. Additionally, the \? command should be able to look up specific help topics. Also, we should make the documentation available via the usql.app website.

This will require effort in a number of ways:

1. Authoring the documentation in a form that can be embedded in both the usql command line application, and used for the website
2. Building the simple documentation site and related infrastructure configuration (like Docusaurus and custom cloud-init scripts)
3. Hooking in embedded documentation to the \? command, as per psql

[nineinchnick]

Does this involve writing docs in comments of exported functions/vars? Or a package comment in doc.go?