[PATCH] doc: outline all planner nodes

Hi,

The docs doesn't currently list all the planner nodes. I wrote a patch to add a new section "14.3" that outlines the different types.

I took the compiled HTML page and hosted it on Netlify to make it easier to peruse:
- https://lovely-lebkuchen-c79301.netlify.app/

Apologies if this isn't formatted normally. This is my first time contributing to the PG project directly.

Best,
Brian B.
Supabase Customer Reliability Engineer

---
 doc/src/sgml/perform.sgml     | 2248 ++++++++++++++++++++++++++++++++-
 doc/src/sgml/ref/explain.sgml |    2 +-
 2 files changed, 2247 insertions(+), 3 deletions(-)