cleanup (sections 0 to 6)

[RMeli]

Cleanup of sections 0 to 6:

• Added comment on f-strings where string formatting was introduced (after the old comment about the new formatting method of Python 3)
  • Started converting to f-strings afterwards, where appropriate
• Extensive use of inline code blocks instead of '' or ""(the correct symbol for a Markdown code block is ``)
• Formatted links
• Removed credits from first notebook since contributions are acknowledged in the README.md file
• Increased header size for exercises

There is now a cleanup.sh script that removes notebook output. It's ideal to use it before commits, so that a pristine state of the notebook is ensured before committ (easier to version control).

Addresses:
#33

[IAlibay]

Couple things, mainly for me to understand so that we have consistent header #s.

[RMeli]

I think I addressed most of the issues. If this PR is ok, I'll do the same with the few remaining sections on my way back.

[IAlibay]

lgtm! 👍