layout folders.subproject new feature (#2662)

* layout folders.subproject new feature

* Update developing_packages/package_layout.rst

Co-authored-by: Luis Martinez <lasote@gmail.com>

* Update developing_packages/package_layout.rst

Co-authored-by: Luis Martinez <lasote@gmail.com>
Co-authored-by: Carlos Zoido <mrgalleta@gmail.com>

developing_packages/package_layout.rst

@@ -538,3 +538,71 @@ The ``conanfile.py`` would look like this:
               # The build() method can also access the CMakeLists.txt in the source folder
               path = os.path.join(self.source_folder, "CMakeLists.txt")
               cmake = load(self, path)
+
+
+Example: Multiple subprojects
+-----------------------------
+
+Lets say that we have a project that contains multiple subprojects, and some of these subprojects need
+to access some information that is at their same level (sibling folders). Each subproject would be 
+a Conan package.
+
+So we have the following folders and files:
+
+.. code-block:: text
+
+    ├── pkg
+    │    ├── conanfile.py
+    │    ├── app.cpp  # contains an #include "../common/myheader.h"
+    │    └── CMakeLists.txt # contains include(../common/myutils.cmake)
+    ├── common
+    │    ├── myutils.cmake
+    │    └── myheader.h
+    └── othersubproject
+
+
+The ``pkg`` subproject needs to use some of the files located inside the ``common`` folder (that might be
+used and shared by other subprojects too), and it references them by their relative location.
+Note that ``common`` is not intended to be a Conan package. It is just some common code that will be copied
+into the different subproject packages.
+
+We can use the ``self.folders.root = ".."`` layout specifier to locate the root of the project, then
+use the ``self.folders.subproject = "subprojectfolder"`` to relocate back most of the layout to the
+current subproject folder, as it would be the one containing the build scripts, sources code, etc.,
+so other helpers like ``cmake_layout()`` keep working.
+
+
+.. code-block:: python
+
+    import os
+    from conan import ConanFile
+    from conan.tools.cmake import cmake_layout, CMake
+    from conan.tools.files import load, copy, save
+
+    class Pkg(ConanFile):
+        name = "pkg"
+        version = "0.1"
+        settings = "os", "compiler", "build_type", "arch"
+        generators = "CMakeToolchain"
+
+        def layout(self):
+            self.folders.root = ".."
+            self.folders.subproject = "pkg"
+            cmake_layout(self)
+
+        def export_sources(self):
+            source_folder = os.path.join(self.recipe_folder, "..")
+            copy(self, "*", source_folder, self.export_sources_folder)
+
+        def build(self):
+            cmake = CMake(self)
+            cmake.configure()
+            cmake.build()
+            self.run(os.path.join(self.cpp.build.bindirs[0], "myapp"))
+
+
+Note it is very important the ``export_sources()`` method, that is able to maintain the same relative layout
+of the ``pkg`` and ``common`` folders, both in the local developer flow in the current folder, but also
+when those sources are copied to the Conan cache, to be built there with ``conan create`` or ``conan install --build=pkg``.
+This is one of the design principles of the ``layout()``, the relative location of things must be consistent in the user
+folder and in the cache.