[generator] MesonDeps (#2654)

* Added MesonDeps information

* Added more info

* Complete meson.build

* Added type of code-block

reference/conanfile/tools/meson.rst

@@ -16,4 +16,5 @@ to try this Meson integration.
    :maxdepth: 2
 
    meson/mesontoolchain
+   meson/mesondeps
    meson/meson

reference/conanfile/tools/meson/mesondeps.rst

@@ -0,0 +1,115 @@
+.. _MesonDeps:
+
+MesonDeps
+=========
+
+.. warning::
+
+    These tools are still **experimental** (so subject to breaking changes) but with very stable syntax.
+    We encourage the usage of it to be prepared for Conan 2.0.
+
+:ref:`MesonToolchain<conan-meson-toolchain>` normally works together with :ref:`PkgConfigDeps<PkgConfigDeps>` to manage all the dependencies,
+but sometimes we need to gather some flags coming from ``Autotools`` tool so that's what ``MesonDeps`` is meant for. In other words, it is typically used
+when Meson cannot find a dependency using the already known `detection mechanisms <https://mesonbuild.com/Dependencies.html>`__ like: `pkg-config`, `cmake`, `config-tool`, etc.
+For instance, if we'd have these lines in your `meson.build` file, you might need ``MesonDeps`` to find that dependency and inject the correct flags to the compiler:
+
+.. code-block:: text
+    :caption: **meson.build**
+
+    project('tutorial', 'cpp')
+    cxx = meson.get_compiler('cpp')
+    mylib = cxx.find_library('mylib', required: true)
+    executable('app', 'main.cpp', dependencies: mylib)
+
+
+In a nutshell, the ``MesonDeps`` generator is the dependencies generator for Meson and GNU flags. It creates a
+`conan_meson_deps_flags.ini` file with all those flags collected by each dependency.
+
+
+.. important::
+
+    At this moment, this generator must be used along with ``MesonToolchain`` one to make it work correctly.
+
+
+.. important::
+
+    This class will require very soon to define both the "host" and "build" profiles. It is very recommended to
+    start defining both profiles immediately to avoid future breaking. Furthermore, some features, like trying to
+    cross-compile might not work at all if the "build" profile is not provided.
+
+
+The ``MesonDeps`` generator can be used by name in conanfiles:
+
+.. code-block:: python
+    :caption: conanfile.py
+
+    class Pkg(ConanFile):
+        generators = "MesonDeps"
+
+.. code-block:: text
+    :caption: conanfile.txt
+
+    [generators]
+    MesonDeps
+
+And it can also be fully instantiated in the conanfile ``generate()`` method:
+
+.. code:: python
+
+    from conan import ConanFile
+    from conan.tools.meson import MesonDeps
+
+    class App(ConanFile):
+        settings = "os", "arch", "compiler", "build_type"
+
+        def generate(self):
+            tc = MesonDeps(self)
+            tc.generate()
+
+The ``MesonDeps`` generates after a ``conan install`` command a `conan_meson_deps_flags.ini` file:
+
+.. code-block:: bash
+
+    [constants]
+    deps_c_args = []
+    deps_c_link_args = []
+    deps_cpp_args = []
+    deps_cpp_link_args = []
+
+
+This generator defines a Meson constants: ``deps_c_args``, ``deps_c_link_args``, ``deps_cpp_args``, ``deps_cpp_link_args``,
+that accumulate all dependencies information, including transitive dependencies, with flags like ``-I<path>``, ``-L<path>``, etc.
+
+.. important::
+
+    Those variables are added automatically as part of the built-in options declared by ``MesonToolchain`` generator: ``c_args``, ``c_link_args``,
+    ``cpp_args``, ``cpp_link_args``.
+
+
+.. note::
+
+    For now, only the ``requires`` information is generated, the ``tool_requires`` one is not managed by this generator yet.
+
+
+Attributes
+++++++++++
+
+* ``c_args``, ``c_link_args``, ``cpp_args``, ``cpp_link_args``: list of flags that accumulate all dependencies information. Each one
+  is saved as ``deps_c_args``, ``deps_c_link_args``, ``deps_cpp_args``, and ``deps_cpp_link_args``, respectively in the
+  `conan_meson_deps_flags.ini` file.
+
+.. code:: python
+
+    from conan import ConanFile
+    from conan.tools.meson import MesonDeps
+
+    class App(ConanFile):
+        settings = "os", "arch", "compiler", "build_type"
+
+        def generate(self):
+            tc = MesonDeps(self)
+            tc.c_args.append("-val1")
+            tc.c_link_args.append("-val2")
+            tc.cpp_args.append("-val3")
+            tc.cpp_link_args.append("-val4")
+            tc.generate()