Nudge new users towards using this style:
```lua
local config = {}
config.color_scheme = 'Batman'
return config
```
and surface how to write lua modules closer to the main section
on config files. In that lua modules section, nudge towards using
a convention similar to that of the plugin spec described in
this commit: e4ae8a844d
This change causes key table activations to effectively layer
over prior key table activations.
This is necessary for the copy mode key assignment changes to
work.
refs: https://github.com/wez/wezterm/issues/993
The keys section was way too big; this splits it up into more
manageable pieces, adds a nice flow chart to show how key events
are processed and adds an example of using the new key tables feature.
