add index or other feature to facilitate readability of `CONTRIBUTING`

[avallecam]

we can use

• table of contents https://github.com/Louis3797/awesome-readme-template#notebook_with_decorative_cover-table-of-contents
• emojis for sections https://github.com/citation-file-format/citation-file-format#contributing-handshake

Accounting that we may need to include more details in #75