doc: Add best practises section to partitioning docs

A few questionable partitioning designs have been cropping up lately
around the mailing lists.  Generally, these cases have been partitioning
using too many partitions which have caused performance or OOM problems for
the users.

Since we have very little else to guide users into good design, here we
add a new section to the partitioning documentation with some best
practise guidelines for good design.

Reviewed-by: Justin Pryzby, Amit Langote, Alvaro Herrera
Discussion: https://postgr.es/m/CAKJS1f-2rx+E9mG3xrCVHupefMjAp1+tpczQa9SEOZWyU7fjEA@mail.gmail.com
Backpatch-through: 10