Add integrations subsection
Adds a TimescaleDB integration tutorial.
Related to #467.
Hm, this is written in a tutorial form, but not sure if this belongs into our Tutorials section.
The important thing is that having done the tutorial, the learner is in a position to make sense of the rest of the documentation, and the software itself.
From https://documentation.divio.com/tutorials/.
So it doesn't belong into Tutorials because it's not really about our software.
Somehow I think it's more of a how-to, other integrations might not necessarily be written in a tutorial form. It would also fit into our Ecosystem section.
Yeah, I think the Integrations subsection should go in Ecosystem.
Superseded by https://github.com/PostgREST/postgrest-docs/pull/619