move clientside_callbacks out of performance and into it's own chapter && create a new "callbacks" section

perhaps we can have a section title that’s like “Advanced Callbacks” and has the following chapters:

1. Clientside Callbacks (pull it out of performance)
2. Determining Which Input Has Changed
3. When to use a single callback vs multiple callbacks
4. Callback limitations
5. callback_context (pull it out of faq) #760
6. Describe initialization and the is None check

With clientside callbacks within performance, it doesn’t lend itself well to searches

[Marc-Andre-Rivet]

@shammamah Callbacks are one of the fundamental aspects of building an app. I think it deserves its own section.

[Marc-Andre-Rivet]

It might also be interesting to move the callbacks Gotchas (https://dash.plot.ly/faqs) in Gotchas/FAQ/Assumptions/Rules under the new callbacks section.

• All callbacks must be defined before the server starts
• A component/property pair can only be the Output of one callback
• Callbacks require all Inputs, States, and Output to be rendered on the page
• Callbacks require their Inputs, States, and Output to be present in the layout
• (already mentioned) Q: How do I determine which Input has changed?