Document new community roles and any rules/guidelines that apply to them

[GuySartorelli]

The maintainer guidelines docs details the CMS Squad and Core Committers, and rules/guidelines that apply to them. We should document the new role(s) similarly.

Notes

• We may decide to hold off on the refiner role for now and only proceed with the reviewer role until we've seen how that goes. In that case, we only need to document the one role for now.

Epic

• EPIC: Empower community members to be more involved in Silverstripe CMS OSS #141

Acceptance Criteria

• New reviewer role is given a proper name and is clearly documented
• New refiner role is given a proper name and is clearly documented
• Documentation includes rules set forth in the proposal document
• Documentation includes process for how to identify, invite, and onboard people to the new roles
• The documentation clearly highlights that it's in a provisional state and is likely to change over time.

PRs

• DOC Document new community roles developer-docs#394

[maxime-rainville]

All done.