[New Content]: Summary paragraph for the PDDL reference page

[rpgoldman]

New Content Proposal:

Add a paragraph briefly summarizing the different PDDL dialects and what the reader should expect from the pages below. I was going to point a novice at this page, but they would have no idea which of the sub-pages they should be looking at -- PDDL, PDDL2.1, PDDL+... None of these says why one would look to one or the other of these dialects.

One possibility (could be combined with a summary paragraph) would be to add a little TL;DR to each of the menu items:

PDDL: the core specification
PDDL 2.1: adds planning with metric time with "durative actions"
PDDL 2.2: honestly, I have no idea
PDDL 3: Add the ability to evaluate plans based on the action sequences they contain, opening the door to new kinds of optimization.
PDDL+: Add the ability to plan for domains that feature not just discrete action sequences, but also continuous dynamics (e.g., pouring fluids at some rate).

Section:
Reference/Guide
PDDL/Planners

Justification:
A less sophisticated audience needs some help navigating this page and knowing why there are these multiple different specs to read.

I note that in theory the PDDL versions include prior versions, but in practice, if you want to read about durative actions, you must read the PDDL 2.1 paper. On the other hand, if you want to do just STRIPS planning, the PDDL 2.1 paper is not helpful because it doesn't focus on conventional classical planning, even though PDDL 2.1 does include all the previous dialects.

This structure is mirrored in the wiki, as one sees, e.g., in the PDDL 2.1 page "This guide will only show features that have been changed or added to PDDL 1.2 in order to form 2.1"