add directive(s) to support grids

[2bndy5]

In mkdocs-material v9.5.0, support for grids was introduced. While upstream requires a third-party mkdocs plugin (& raw HTML for <div> container), we could simply use a combination of rst-class and container directives.

rST equivalent to material MD

.. rst-example:: Card grid

    .. rst-class:: grid cards

        .. container::

            - :si-icon:`fontawesome/brands/html5` **HTML** for content and structure
            - :si-icon:`fontawesome/brands/js` **JavaScript** for interactivity
            - :si-icon:`fontawesome/brands/css3` **CSS** for text running out of boxes
            - :si-icon:`fontawesome/brands/internet-explorer` **Internet Explorer** ... huh?

.. rst-example:: Card grid, blocks

    .. rst-class:: grid

        .. container::

            .. rst-class:: card

            :si-icon:`fontawesome/brands/html5` **HTML** for content and structure

            .. rst-class:: card

            :si-icon:`fontawesome/brands/js` **JavaScript** for interactivity

            .. rst-class:: card

            :si-icon:`fontawesome/brands/css3` **CSS** for text running out of boxes

                :si-icon:`fontawesome/brands/internet-explorer` **Internet Explorer** ... huh?

.. rst-example:: Generic grid

    .. rst-class:: grid

        .. container::

            .. md-tab-set::

                .. md-tab-item:: Unordered list

                    * Sed sagittis eleifend rutrum
                    * Donec vitae suscipit est
                    * Nulla tempor lobortis orci

                .. md-tab-item:: Ordered list

                    1. Sed sagittis eleifend rutrum
                    2. Donec vitae suscipit est
                    3. Nulla tempor lobortis orci

            .. code-block:: rst
                :caption: Content tabs

                .. md-tab-set::

                    .. md-tab-item:: Unordered list

                        * Sed sagittis eleifend rutrum
                        * Donec vitae suscipit est
                        * Nulla tempor lobortis orci

                    .. md-tab-item:: Ordered list

                        1. Sed sagittis eleifend rutrum
                        2. Donec vitae suscipit est
                        3. Nulla tempor lobortis orci

It would nicer to have a directive that acts as syntactic sugar to automatically achieve this. For inspiration, sphinx-design has a grid and grid-tem directives, although they are a bit more deterministic than upstream's implementation.