Add CLI page

[steve-chavez]

The CLI is undocumented right now.

We need this page to document #2856.

There's some extra work here:

• --dump-schema should be called --dump-schema-cache. Or perhaps --dump-scache as an alias.
• --example only shows a config file example. It should show env vars and maybe in-db config.
• A mention about using --dump-schema with jq.

[laurenceisla]

An idea for --example: maybe adding parameters could get different configs, e.g. --example CONFIGTYPE, where CONFIGTYPE may be one of 'file', 'env', 'db' or something like that.

[wolfgangwalther]

I transferred this issue back from the postgrest-docs repo. It didn't really fit into any of the other "collection" issues - it is in fact the only issue that is a clear cut "missing" in the references section. That deserves a separate issue, I guess.