docs(contributing) revamp with code style

A more welcoming contributing guide, addressed to both members of the
community (technical or not, seeking for help or to contribute), and new
hires.

This includes an updated code style, an FAQ about which channels to
communicate on, a contributing guide with guidelines for good commit
messages and overall healthy contributions.

This replaces Kong#2253 which became style, and introduces a new, undesired
`CODE_SYLE.md` file.