https://issues.liferay.com/browse/LRDOCS-1286

https://issues.liferay.com/browse/LRDOCS-1286

Updated Markdown Converter to handle sidebars with multiple paragraphs; added a new Markdown syntax for sidebars:

Use $beginSBnote$ to begin a sidebar note and $endSBnote$ to end it. Similarly for $beginSBtip$, $endSBtip$,  $beginSBwarning$, and $beginSBwarninge$. These tokens must appear as Markdown paragraphs: no other content on the same line and blank lines above and below the tokens. All the Markdown paragraphs between the begin and end tokens will appear in the sidebar.

Tested with multiple paragraphs in sidebars. Once we determine a final syntax for the sidebars, we can remove the old sidebar handler from the Markdown converter since we don't want to specify images for sidebars in the Markdown source.

@sez11a