Support Markdown content for Sidebar tag

[alexander-rabenstein]

Currently the sidebar element support only direct confluence markup, without any break. If you want to have complex content in the sidebar , you need to remove any linebreaks, for example:

<!-- Sidebar: <h3>Sidebar content</h3><ac:structured-macro ac:macro-id="008e6671-7aa1-4696-b231-cf7d38b4df17" ac:name="panel" ac:schema-version="1"><ac:rich-text-body><p>Changelog:</p><table><colgroup><col/><col/></colgroup><tbody><tr><th>Date</th><th>Change</th></tr><tr><td><br/></td><td><br/></td></tr><tr><td><br/></td><td><br/></td></tr></tbody></table></ac:rich-text-body></ac:structured-macro> -->

Either a possibility needs to be added to include a different markdown file as the sidebar content, or to use linebreaks and markdown inside of the sidebar tag.