Add documentation regarding internal render pipeline and and update flow

[etimberg]

As discussed in the comments of #4468, it is helpful to have documentation on how Chart.js works internally.

Some options / ideas:

1. Config merge process
2. Layout system & fitting algorithm
3. Chart controllers and meta data
4. Render pipeline and how animations are handled

[Riotsmurf2]

Assuming this won't ever happen? I am trying to use this great library but end up having to go to other websites to find simple things that could just be in a reference document. For example, no where on your documentation site does it show fontColors as an option for ticks, but there is. I had to google it. This library is very frustrating to use when no ALL the option references are available to me in the documentation itself.

[etimberg]

@Riotsmurf that particular option is documented at https://www.chartjs.org/docs/latest/axes/styling.html#tick-configuration

In terms of writing more detailed internal documentation, there has not been any progress.

[Riotsmurf2]

Literally just came back here to say i found it. I suppose i just needed to stare at it for a bit.
cheers