Make style changes to contributing file

[ExcaliburZero]

• Demote most of the headings one level, as h1 is generally used for the title of a page.
• Change letter based sub lists to be unordered lists, as letter-based lists are not rendered in GitHub markdown.
• Make a few fixes to the "Adding language support" section.
• Change all text-based emoji to GitHub-supported emoji.
• Fix broken link in the "Contributing code" section.

[M-Zuber]

:+1: Thank you :smile:

[basicallydan]

:+1: Great stuff, cheers.