chore: propose code of conduct

[moonmeister]

Propose and add a code of conduct to the repo. There are many good ones we can use or pull from.

• https://www.contributor-covenant.org/
• https://github.com/withastro/.github/blob/main/CODE_OF_CONDUCT.md

[colinmurphy]

Adding a few examples for reference:

Gatsby - https://www.gatsbyjs.com/contributing/code-of-conduct/ (linked from https://github.com/gatsbyjs/gatsby/blob/master/CODE_OF_CONDUCT.md)

WP GraphQL - https://github.com/wp-graphql/wp-graphql/blob/develop/.github/CODE_OF_CONDUCT.md (note that this makes a tab on the project page)

Symfony - https://symfony.com/doc/current/contributing/code_of_conduct/code_of_conduct.html (linked from https://github.com/symfony/symfony/blob/7.3/CODE_OF_CONDUCT.md)

Github Open Source - https://github.com/github/opensource.guide/blob/main/CODE_OF_CONDUCT.md (note - https://opensource.guide/code-of-conduct/)

[colinmurphy]

Draft PR - #105

[colinmurphy]

Closed by #105