Fix html in codeblocks and dash rendering in markdown

[daexs]

• Converted escaped HTML (eg. <) into raw HTML.
• Made minor line break and spacing changes to prevent code blocks from being too long and requiring scrolling to the right to view the full line for HTML code blocks..
• Reformatted code blocks that previously rendered iframes for Dash examples to properly display the code snippets, charts and hide the code block that retrieves the code snippets.
• Deleted previous custom formatter to hide these code blocks and removed the configuration from mkdocs.yml and Makefile.

Closes #62 and #45