Support include and literal include directives with use mkdocs-include-markdown plugin

[jodygarnett]

GeoServer makes use of include and literalinclude directives, map these to mkdocs-include plugin use.

Although material-for-mkdocs recommends use of snippets the mkdocs-include plugin closely matches the sphinx-build functionality.

Implementation notes:

• Use of absolute path is assumed from docs folder
• Use ./ or ../ to indicate a relative path

Reference:

• https://github.com/mondeja/mkdocs-include-markdown-plugin

literal include directive mapping

Used to include code example:

RichStructuredText:

.. literalinclude:: files/counteries.sld
   :language: xml

Markdown:

 ```xml
 {% include "./files/countries.sld" %}
 ```

include directive markdown

Used to include rst content inline:

RichStructuredText

.. include:: /../../../../LICENSE.md
   :start-after: GeoServer is distributed under the GNU General Public License Version 2.0 license:
   :end-before: For latest contact information of Open Source Geospatial Foundation see the website at

MarkDown:

{%
   include-markdown "../../../../../LICENSE.md"
   start="GeoServer is distributed under the GNU General Public License Version 2.0 license:"
   end="For latest contact information of Open Source Geospatial Foundation see the website at"
   dedent=true
%}