Documentation: Add start of a Porting guide

Author: https://github.com/ADKaster
Commit: https://github.com/LadybirdBrowser/ladybird/commit/3b2b86303f7
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/1718
Reviewed-by: https://github.com/AtkinsSJ
Reviewed-by: https://github.com/LucasChollet

Documentation/Porting.md

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+# Porting to new Operating System Environments
+
+This document outlines the steps required to port Ladybird to a new OS environment, and expectations for such ports to
+be accepted into the mainline repository.
+
+## Types of Ports
+
+There are two types of ports that can be made to Ladybird:
+
+- Chrome port: We define the "browser chrome" as the UI layer. This includes the browser window, tabs, address bar, etc.
+- Platform port: This includes the underlying platform-specific code that interacts with the OS. This includes things like
+  file I/O, networking, and process management.
+
+### Chrome Ports
+
+There are currently three supported chrome ports:
+
+- Qt6: The generic chrome port.
+- AppKit/Cocoa: The macOS native port, which uses the AppKit framework.
+- Headless: A headless port that does not have a UI, used for testing.
+
+### Platform Ports
+
+There are currently two supported platform ports:
+
+- GNU/Linux: The Linux platform port that may work on other POSIX platforms
+- macOS: The macOS platform port
+
+Many other POSIX desktop platforms are known to work, or have worked in the past, but are not officially supported.
+These include Alpine Linux, FreeBSD, OpenBSD, and Haiku. Support for these Unixen is community-supported, meaning there
+is no regular CI guaranteeing that each master commit keeps them working. Patches to bring them back in line
+are welcome!
+
+There is currently one in progress platform port:
+
+- Android: The Android platform port, using the Android SDK/NDK directly.
+
+## Porting Steps
+
+### Chrome ports
+
+Chrome ports mostly concern themselves with the UI layer. This means the main Ladybird process, using LibWebView.
+
+To create a new Ladybird chrome, you will need to implement a new `WebView::ViewImplementation` subclass.
+ViewImplementation is the main interface between the UI process and WebContent processes. It is expected that each tab
+of the browser will have its own WebContent process. This is all managed by the WebView layer.
+
+Each chrome must also subclass `WebView::Application` to add any chrome-specific command-line flags.
+
+TODO: Explain any more details that are necessary
+
+### Platform ports
+
+Platform ports concern themselves with the underlying OS-specific code. In Ladybird, this code is largely contained in
+the AK and LibCore libraries.
+
+AK is the standard template library for Ladybird. The first step of a new platform port is a new platform define in
+`AK/Platform.h`. This define will be used to conditionally compile platform-specific code. 
+In AK, the most likely class to need platform-specific code is `AK::StackInfo`.
+
+LibCore is an abstraction over POSIX. It contains classes to wrap lower level OS functionality into APIs that are
+comfortable for Ladybird developers to use. The most likely place to need adjustment is `Core::System`, followed by
+`Core::Process` and `Core::Socket`.
+
+Ladybird makes heavy use of IPC, and the IPC layer is in `LibIPC`. This layer is mostly platform-agnostic, but there are
+some platform-specific details in LibCore that may need to be adjusted. The IPC system is based on Unix domain sockets,
+so  any platform that supports Unix domain sockets should be able to use the IPC system out of the box.
+
+## Special Consideration
+
+### Windows
+
+Over the years excitement about a native windows port has waxed and waned. The main issue is that Ladybird is built on
+top of LibCore, which is a POSIX abstraction. Windows is not POSIX, and so a Windows port requires a significant amount
+of effort to implement.
+
+To limit the scope of the work, we have decided that a Windows port will need the following properties:
+
+- Target x86_64-windows-msvc and arm64-windows-msvc only. We are not interested in accepting patches to make MinGW or MSYS2 work.
+- The port must use `clang-cl.exe` as the compiler. We are not interested in accepting patches to make MSVC work.
+- Platform `#ifdef`s must stay within AK and LibCore, with a few exceptions in LibWebView.
+- Prefer separate cpp files with the same interface to `#ifdef`s within a single file.
+- Avoid `#ifdef`s in headers, as much as possible.