Move codepen samples from Front Matter to content

This allows us to place our own content between codepen samples
Merge pull request #1 from matthewdunsdon/enhancement/allow-post-to-position-codepens

Allow 'fundamentals' sample page to create its own codepens
Fixed post after accidental codepen url change
Add initial outline to 'fundamentals' post

- Explain the areas I would like to discuss
- Included placeholders for proposed codepens
Update sample styles for paragraphs

Previously, it was unclear where one paragraph ended and another started.
Partial update of 'fundamentals' sample page

Started adding codepens and updating some of the prose.
Increase codepen heights

Some codepen examples were chopping off new code
Completed fundamentals samples page

- Split the original page into several parts
- Sample index page orders links order by post date