Updating SlotFill documentation and examples

[ryanwelcher]

Description

This PR aims to improve the documentation and examples for SlotFill based on work done in https://github.com/10up/slotfill-and-filter-demos

Types of changes

Documentation

Checklist:

• I've included developer documentation if appropriate.

[oandregal]

These docs are great, thanks @ryanwelcher!

I was thinking, though: what if we add this to a new section in the handbook called "Slot Reference"? I believe it would give them more visibility: it is an important concept that isn't covered in the reference docs yet although we do have a tutorial and component docs. It'd fit well just after the "Filter Reference" section almost verbatim, and the "existing slots" could be subsections that we'd add as new slots are implemented.

What do you think?

[ryanwelcher]

@nosolosw that would work for me. One of the issues I'm running into is adding an image for each of the slotsFills because the docs get auto-generated.

I've started adding them into the docs now and should have this PR updated soon.

[ryanwelcher]

@nosolosw thanks for the review and suggestions on this!
I have merged your changes, updated the toc.json file, and rebuilt the manifest.

[oandregal]

Let's get this in and follow-up on the block-editor SlotFills. I'm merging this as soon as Travis would let us.

[oandregal]

This is a great improvement to the handbook, thanks for getting this in @ryanwelcher ! 🌟

[youknowriad]

Awesome work here 👍

[ryanwelcher]

Glad to help :)