Devdocs reorganisation (#913)

* docs: split usage and dev docs

* # This is a combination of 2 commits.
# This is the 1st commit message:

docs: split usage and dev docs

# The commit message #2 will be skipped:

# fixup add docs

* docs: add runner documentation and move hooks documentation to devdocs

* docs: add stubs for modules technical description

* docs: add paragraph about event thread-safety

* docs: add 'Current modules' section header

src/runner/README.md

@@ -1,50 +0,0 @@
-# PowerToys Runner
-
-# Introduction
-
-The PowerToys Runner contains the project for the PowerToys.exe executable.
-It's responsible for:
-- Loading the individual PowerToys modules.
-- Passing registered events to the PowerToys.
-- Showing a system tray icon to manage the PowerToys.
-- Bridging between the PowerToys modules and the Settings editor.
-
-![Image of the tray icon](/doc/images/runner/tray.png)
-
-# Code organization
-
-#### [`main.cpp`](./main.cpp)
-Contains the executable starting point, initialization code and the list of known PowerToys.
-
-#### [`powertoy_module.h`](./powertoy_module.h) and [`powertoy_module.cpp`](./powertoy_module.cpp)
-Contains code for initializing and managing the PowerToy modules.
-
-#### [`powertoys_events.cpp`](./powertoys_events.cpp)
-Contains code that handles the various events listeners, and forwards those events to the PowerToys modules.
-
-#### [`lowlevel_keyboard_event.cpp`](./lowlevel_keyboard_event.cpp)
-Contains code for registering the low level keyboard event hook that listens for keyboard events.
-
-#### [`win_hook_event.cpp`](./win_hook_event.cpp)
-Contains code for registering a Windows event hook through `SetWinEventHook`, that listens for various events raised when a window is interacted with.
-
-#### [`tray_icon.cpp`](./tray_icon.cpp)
-Contains code for managing the PowerToys tray icon and its menu commands.
-
-#### [`settings_window.cpp`](./settings_window.cpp)
-Contains code for starting the PowerToys settings window and communicating with it.
-
-#### [`general_settings.cpp`](./general_settings.cpp)
-Contains code for loading, saving and applying the general setings.
-
-#### [`auto_start_helper.cpp`](./auto_start_helper.cpp)
-Contains helper code for registering and unregistering PowerToys to run when the user logs in.
-
-#### [`unhandled_exception_handler.cpp`](./unhandled_exception_handler.cpp)
-Contains helper code to get stack traces in builds. Can be used by adding a call to `init_global_error_handlers` in [`WinMain`](./main.cpp).
-
-#### [`trace.cpp`](./trace.cpp)
-Contains code for telemetry.
-
-#### [`svgs`](./svgs/)
-Contains the SVG assets used by the PowerToys modules.