Section on debugging SELECT performance

[mfreed]

When users encounter slower-than-expected query performance, we often see that it's attributed to a set of common reasons. We should add a section around "best practices" or "what to check if your queries are slow" to the docs.

Find below a draft outline of this section; posting as a github issue so that the community could offer their own suggestions before we finalize the docs pages.

[cocowalla]

The guidance added for INSERT performance is absolute gold 🥇 - really looking forward to guidance on SELECT perf 🙏