Docs: Refactor/review linked guidance usage, and move jsonschema directive to end of sections

[yolile]

At the same time we can make the changes suggested in #1434 (comment):

...we want all content except the extensions list above the jsonschema directive, as otherwise it's easy to miss.)

I think we might start using a simpler format of "seealso ... How to [do X]" when linking to guidance.

Originally posted by @duncandewhurst in #1297 (comment)

We can search for "For more information". In many cases (not all), seealso would be better. There are also some {note} that start with For that can be changed to seealso.

[duncandewhurst]

@jpmckinney happy for me to make these changes as part of #1711?

[jpmckinney]

Sure, though #1660 (and to a small extent #1680) make changes to schema/reference.md, so #1711 might end up with big conflicts if you add too much to it before those are merged.