GODT-1676: developer documentation. [skip ci]

Other: bridge-gui readme. [skip ci]

internal/frontend/bridge-gui/README.md

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+# bridge-gui
+
+bridge-gui is the graphical user interface for Bridge. It's a C++ [Qt](https://www.qt.io) application  that 
+communicates with the bridge executable via gRPC remote procedure call, on a local-only TLS-secured connection. 
+For more technical details ,see this [RFC](https://confluence.protontech.ch/x/diWHBQ).
+
+# Components
+
+bridge-gui consists in 3 components:
+
+- **bridge-gui**: The Bridge-GUI application itself. It's a QML Qt application that implements the bridge 
+[gRPC](https://www.grpc.io) service to communicate and interact with the bridge application written in 
+[Go](https://go.dev).
+- **bridge-gui-tester**: A Qt widgets test application that offers a dummy gRPC server implementing the Bridge gRPC 
+service. It can be used as a debugging and development tool, as it can simulate the server side (bridge) portion of 
+the gRPC service.
+- **bridgepp**: bridgepp (for bridge++) is a C++ static library that contains the code shared by bridge-gui and 
+bridge-gui-tester.
+
+# Bridge gRPC service
+
+The bridge gRPC service that allows communications between bridge-gui, and the bridge Go application (or 
+bridge-gui-tester) is defined in `../grpc/bridge.proto`.
+
+# Supported platforms
+
+bridge-gui runs on the same platforms as the bridge app:
+
+- Linux x64.
+- macOS x64 and Apple Silicon.
+- Windows x64.
+
+# Requirements:
+
+- A C++ development toolchain ([GCC](https://gcc.gnu.org)/[Clang](https://clang.llvm.org)/
+[MSVC](https://docs.microsoft.com/en-us/cpp/?view=msvc-170), [CMake](https://cmake.org), 
+[Ninja](https://ninja-build.org)). An easy way to get all the needed tools is to use a modern IDE such 
+as [CLion](https://www.jetbrains.com/clion/), [Qt Creator](https://www.qt.io/product/development-tools) or 
+[Visual Studio Code](https://code.visualstudio.com) that will check and install or use their bundled versions
+of the tools.
+- [Qt 6](https://www.qt.io/download-open-source). Use the online installer to install the latest stable 
+release of Qt for your platform and compiler.
+
+# First build
+
+bridge-gui uses [vcpkg](https://vcpkg.io/en/index.html), Microsoft multi-platform C/C++ dependency manager to get 
+the source code and build gRPC and its dependencies (protobuf, zlib, ...). vcpkg is managed as a git submodule 
+in `extern/vcpkg`,relative to the root of the bridge source tree. A pair of scripts is provided to perform 
+the initialization of vcpkg, the retrieval and compilation of gRPC and a first build of the bridge-gui project
+components:
+
+- `build.sh`: a shell script to use on macOS and Linux.
+- `build.ps1`: a [PowerShell](https://docs.microsoft.com/en-us/powershell/) script for Windows.
+