Create pipeline/filter converting ADF to markdown

using kramdown

- Initial attmpt at using kramdown
for ADF to kramdown conversion
- Additional specs for marks code and link
- Add support for strikethrough, superscipt
and subscript
- Add underline and textColor
- Added blockquote, bull_list, heading, and rule
- Add inlineCard and orderedList support
- Added support for panel
- Add codeBlock support
- Added hardBreak and initial table support
- Add emoji support
- Add icons to panels
- Add support for mediaSingle and mediaGroup
- Add support for mentions
- Fix mention and complex document md
- Add handling of tables
- Remove unnecessary guard
- Added specs for invalid situations

Add link to CommonMark spec for clarification

and other minor changes

Use `add_text` method

and other minor changes

Use a blockquote for panels

instead of <hr> separation

Correctly set the raw test for headings

Adjust how mentions are generated

and changes for other review comments

Return original source for invalid documents