Refactor and update content sharing documentation

[choldgraf]

This updates our user documentation around Jupyter Books and nbgitpuller. It has the following goals:

• Separate out "shared documentation" and "sharing content files" into two sections described by their use-cases.
• Unify the Jupyter Book documentation to be consistent in referring to the MyST stack
• Add more section headers that all follow a "how do I XX" type of structure
• Do slight rearranging to make content sharing more discoverable

[agoose77]

In case someone else reviews this first, I'm also reviewing jupyter-book/mystmd#2046