Update contributor docs to recommend tests & docs with each new feature

[terriko]

We currently request documentation and tests as part of code review, but don't have that expectation documented as part of our new contributor documentation. Typically, for shorter code we get the tests/docs added as part of the initial PR, for longer items sometimes a separate issue and a new PR are opened at a later date.

It would be nice to have these expectations documented more comprehensively as part of our new contributor documentation.

There's probably some related stuff that could be done at the same time:

• Renaming the contributor guide: Moving contributor guide to expected location #1446
• Adding guidance about commit messages using conventional commits Update contributor docs to talk about conventional commits #1317