docs: Add Nix flake development environment guide

This commit introduces a new guide in  that details how to set up a reproducible development environment for NuttX using Nix flakes.

The guide covers:
- Prerequisites for using Nix flakes.
- Steps to enter the NuttX development shell.
- Benefits of using the Nix flake (reproducibility, simplified onboarding, dependency management).
- An overview of the  contents.

A link to this new guide has also been added in index.rst to ensure discoverability.

Signed-off-by: Côme VINCENT <44554692+comejv@users.noreply.github.com>

Documentation/guides/index.rst

@@ -7,6 +7,7 @@ Guides
 
 .. toctree::
   nfs.rst
+  nix_flake.rst
   usbtrace.rst
   simulator.rst
   rndis.rst

Documentation/guides/nix_flake.rst

@@ -0,0 +1,88 @@
+======================================
+Nix Flake for Reproducible Development
+======================================
+
+This guide explains how to use the Nix flake to set up a reproducible development environment for NuttX. The Nix flake ensures that all required build tools and dependencies are consistently available, simplifying onboarding and reducing "works on my machine" issues.
+
+Prerequisites
+-------------
+
+*   `Nix <https://nixos.org/download/>`_ installed on your system.
+*   Nix flakes enabled (add ``experimental-features = nix-command flakes`` to your ``nix.conf``).
+
+Setting up the Development Environment
+--------------------------------------
+
+To enter the NuttX development shell, navigate to the root of the NuttX directory and run:
+
+.. code-block:: bash
+
+    nix develop
+
+This command will:
+
+*   Download and set up all necessary build tools and dependencies, including:
+    *   CMake, Ninja, GNU Make
+    *   Clang tools
+    *   ARM toolchain (gcc-arm-embedded)
+    *   Automake, Bison, Flex, Genromfs, Gettext, Gperf
+    *   Kconfig-frontends, libelf, expat, gmp, isl, libmpc, mpfr, ncurses, zlib
+    *   Python with kconfiglib
+*   Set the ``CMAKE_EXPORT_COMPILE_COMMANDS`` environment variable to ``ON``.
+*   Display a welcome message.
+
+Once inside the development shell, you can proceed with building NuttX as usual.
+
+Benefits
+--------
+
+*   **Reproducibility:** Ensures a consistent build environment across all developers and machines.
+*   **Simplified Onboarding:** New contributors can quickly set up their development environment with a single command.
+*   **Dependency Management:** All dependencies are managed by Nix, avoiding conflicts with system-wide packages.
+
+Contents of the Nix Flake
+-------------------------
+
+The `flake.nix` file defines a `devShell` that includes the following build inputs:
+
+.. code-block:: nix
+
+    buildInputs = [
+      # Build tools
+      pkgs.cmake
+      pkgs.ninja
+      pkgs.gnumake
+      pkgs.clang-tools
+
+      # ARM toolchain
+      pkgs.gcc-arm-embedded
+
+      # NuttX dependencies
+      pkgs.automake
+      pkgs.bison
+      pkgs.flex
+      pkgs.genromfs
+      pkgs.gettext
+      pkgs.gperf
+      pkgs.kconfig-frontends
+      pkgs.libelf
+      pkgs.expat.dev
+      pkgs.gmp.dev
+      pkgs.isl
+      pkgs.libmpc
+      pkgs.mpfr.dev
+      pkgs.ncurses.dev
+      pkgs.zlib
+      pkgs.python313Packages.kconfiglib
+    ];
+
+The `shellHook` sets up the `CMAKE_EXPORT_COMPILE_COMMANDS` and provides a welcome message:
+
+.. code-block:: nix
+
+    shellHook = ''
+      export CMAKE_EXPORT_COMPILE_COMMANDS=ON
+      echo "Welcome to NuttX devShell"
+    '';
+
+This setup ensures that the development environment is fully configured for NuttX development.