Add the markdown entry

Summary:
Add the markdown entry to Cantor. Markdown support enables users to
write a document with runnable code in Markdown style. This
implementation supports LaTeX just like the text entry.

A third-party library Discount is linked in order to convert
markdown into HTML.

Update CMakeLists.txt

A screenshot for preview:
{F6186425}

Reviewers: pino, #cantor, filipesaraiva

Reviewed By: #cantor, filipesaraiva

Subscribers: sirgienko, filipesaraiva, pino, asemke, kde-edu

Tags: #kde_edu

Maniphest Tasks: T9108

Differential Revision: https://phabricator.kde.org/D14738