Crate-docs-theme

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- Removed Python 2 in favour of Python 3.
- Removed setuptools requirement.

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- Link to the IoT Data Platform docs is hidden unless you are currently viewing
those docs.

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- Added new CrateDB Cloud IoT Data Platform docs project. This includes a new
template config module and a change to the HTML navigation menu.

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- Unreleased

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- Change the ``html_context`` keys for custom js/css scripts from
``script_files`` to ``extra_script_files`` and from ``css_files`` to
``extra_css_files``.
This change fixes a regression that was introduced in ``0.5.73`` which
allowed the build process on RTD to "inject" their css/js using the
``script_files``/``css_files`` keys of the html context.

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- Change depth of toc tree of Cloud CLI project to 2.

- Allow per-project additional script files by specifying ``script_files`` in
the project's ``html_context`` (in ``conf.py``).

- Allow per-project additional css files by specifying ``css_files`` in
the project's ``html_context`` (in ``conf.py``).