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

doc/devdocs/readme.md

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+# Code Organization
+
+## Rules
+
+- **Follow the pattern of what you already see in the code**
+- Try to package new ideas/components into libraries that have nicely defined interfaces
+- Package new ideas into classes or refactor existing ideas into a class as you extend
+
+## Code Overview
+
+General project organization:
+
+#### The [`doc`](/doc) folder
+Documentation for the project, including a [coding guide](/doc/coding) and [design docs](/doc/specs).
+
+#### The [`installer`](/installer) folder
+Contains the source code of the PowerToys installer.
+
+#### The [`src`](/src) folder
+Contains the source code of the PowerToys runner and of all of the PowerToys modules. **This is where the most of the magic happens.**
+
+#### The [`tools`](/tools) folder
+Various tools used by PowerToys. Includes the Visual Studio 2019 project template for new PowerToys.
+
+# Implementation details
+
+### [`Runner`](/doc/devdocs/runner.md)
+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)
+
+### [`Interface`](/doc/devdocs/modules/interface.md)
+Definition of the interface used by the [`runner`](/src/runner) to manage the PowerToys. All PowerToys must implement this interface.
+
+### [`Common`](/doc/devdocs/common.md)
+The common lib, as the name suggests, contains code shared by multiple PowerToys components and modules, e.g. [json parsing](/src/common/json.h) and [IPC primitives](/src/common/two_way_pipe_message_ipc.h).
+
+
+### [`Settings`](/doc/devdocs/settings.md)
+WebView project for editing the PowerToys settings.
+
+The html portion of the project that is shown in the WebView is contained in [`settings-html`](/src/settings/settings-heml).
+Instructions on how build a new version and update this project are in the [Web project for the Settings UI](./settings-web.md).
+
+While developing, it's possible to connect the WebView to the development server running in localhost by setting the `_DEBUG_WITH_LOCALHOST` flag to `1` and following the instructions near it in `./main.cpp`.
+
+### [`Settings-web`](/doc/devdocs/settings-web.md)
+This project generates the web UI shown in the [PowerToys Settings](/src/editor).
+It's a `ReactJS` project created using [UI Fabric](https://developer.microsoft.com/en-us/fabric#/).
+
+## Current modules
+### [`FancyZones`](/doc/devdocs/modules/fancyzones.md)
+The FancyZones PowerToy that allows users to create custom zones on the screen, to which the windows will snap when moved.
+
+### [`PowerRename`](/doc/devdocs/modules/powerrename.md)
+PowerRename is a Windows Shell Context Menu Extension for advanced bulk renaming using simple search and replace or more powerful regular expression matching.
+
+### [`Shortcut Guide`](/doc/devdocs/modules/shortcut_guide.md)
+The Windows Shortcut Guide, displayed when the WinKey is held for some time.
+
+### _obsolete_ [`example_powertoy`](/doc/devdocs/modules/example_powertoy.md)
+An example PowerToy, that demonstrates how to create new ones. Please note, that this is going to become a Visual Studio project template soon.
+
+This PowerToy serves as a sample to show how to implement the [PowerToys interface](/src/modules/interface/) when creating a PowerToy. It also showcases the currently implemented settings.
+
+#### Options
+This module has a setting to serve as an example for each of the currently implemented settings property:
+  - BoolToggle property
+  - IntSpinner property
+  - String property
+  - ColorPicker property
+  - CustomAction property
+
+![Image of the Options](/doc/images/example_powertoy/settings.png)