Fix doxygen markdown rendering

Summary:
Doxygen's parser appears to have trouble with code blocks delimited by
backticks and lines within the code blocks that end with backticks.

This also fixes an issue where a heading isn't rendered correctly
unless it has a blank line after it.

Test Plan: built docs locally

Reviewers: apol

Reviewed By: apol

Subscribers: apol, kde-frameworks-devel, kde-doc-english

Tags: Frameworks, Documentation

Differential Revision: https://phabricator.kde.org/D17466