Improve explanation of patches in V1 Plugin API docs

[kdmccormick]

Background

Based on this Slack conversation: https://openedx.slack.com/archives/C02V3GHE3UP/p1655384231694799

Essentially, the v0 API docs put the concept of "patches" front-and-center, and had a nice little explanation of them. The equivalent V1 docs for patches are nested under Reference -> Hooks API -> Constants, and they are very basic. There is a Template Patches reference page, but it does not explain patches, it just lists them.

Tasks

Make it so it's easier for new plugin developers to understand what a patch is. This could involve adding a new docs section, improving the existing ones, or both.

[MaferMazu]

I can work on this @kdmccormick

[MaferMazu]

I started this work here: overhangio/tutor#852