Guide explaining how to keep compatibility with mkdocs

Author: humitos

docs/guides/mkdocs-old-versions.rst

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+Keep Building Docs With Old Version Of MkDocs
+=============================================
+
+Recent changes on ``mkdocs`` forced us to `upgrade the default version installed`_ by Read the Docs and this may be a breaking change for your documentation.
+
+.. _upgrade the default version installed: https://github.com/rtfd/readthedocs.org/pull/4041
+
+You should check that your docs continue building in any of these cases:
+
+* your project doesn't have a ``requirements.txt`` file pinning ``mkdocs`` to a specific version
+* your project is using a custom theme
+* your project is using Markdown extensions
+
+In case your builds are failing because of a ``mkdocs`` issue,
+you may want to follow one of the following solutions depending on the case.
+
+
+Pin mkdocs to the previous version
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Before Read the Docs upgraded its default version installed, ``mkdocs==0.15.0`` was used.
+To make your project continue using this version you will need to create a ``requirements.txt`` file with this content::
+
+     # requirements.txt
+     mkdocs==0.15.0
+     mkdocs-bootstrap==0.1.1
+     mkdocs-bootswatch==0.1.0
+
+.. note::
+
+   Most of the breaking changes where introduced in ``mkdocs==0.17``,
+   so you may want to test only pinning ``mkdocs`` to ``mkdocs<0.17``
+   and check if your docs keep building successfully.
+
+
+Upgrade your custom theme to be compatible with newer mkdocs versions
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+It is possible that event your build passes your documentation doesn't look correct.
+This may be because the new ``mkdocs==0.17.3`` version installed by Read the Docs introduced some breaking changes on the structure of the theme.
+
+You should check the `mkdocs' Custom Theme documentation`_ to upgrade your custom theme and make it compatible with the new version.
+
+.. _mkdocs' Custom Theme documentation: https://www.mkdocs.org/user-guide/custom-themes/
+
+
+Upgrade how extension arguments are defined
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+``mkdocs`` has changed the way that ``markdown_extensions`` are defined and you may need to upgrade it.
+If you where passing arguments to the extension by defining them between brackets (``toc(permalink=true)``) in your ``mkdocs.yaml`` you may need to upgrade it to the new way.
+
+For example, this definition:
+
+.. code-block:: yaml
+
+   markdown_extensions:
+     - admonition
+     - codehilite(guess_lang=false)
+     - toc(permalink=true)
+     - footnotes
+     - meta
+
+needs to be replaced by:
+
+.. code-block:: yaml
+
+   markdown_extensions:
+     - admonition
+     - codehilite
+         guess_lang: false
+     - toc
+         permalink: true
+     - footnotes
+     - meta