Document common patterns

[Akuli]

From #1308

I would like to see more patterns documented, it's pretty daunting trying to get started.

For instance, I'm looking at the dedent method of the MainText widget.

There is a lot to figure out here. If there was a "cookbook" of common patterns that would make getting started a lot less daunting.

Making a list of stuff to document...

text widget specific:

• how to get line and column numbers in text widget, e.g. int(self.index(location).split(".")[1])
• how to construct Tk's text widget location strings, e.g. line = self.get(f"{location} linestart", f"{location} lineend")
• the "end - 1 char" madness

Other stuff:

• how to access settings, e.g. self._tab.settings.get("indent_size", int)
• .update() + .event_generate() in tests (from code review with benjamin-kirkbride)
• boilerplate of new plugin: on_new_filetab() and the like
• virtual events (already documented, but should link to virtual events doc because they are a very common pattern)
• probably many things from docs/plugin-intro.rst

Please suggest more stuff to be added here :)