Maintainer docs say peer reviewers can merge PRs, but that's no longer feasible for security reasons

[GuySartorelli]

https://docs.silverstripe.org/en/5/project_governance/maintainer_guidelines/ says community peer reviewers will have the rights to merge pull requests.

Unfortunately for security reasons (see silverstripe/.github#260) that's not feasible.

Update the docs to reflect that core committers and CMS Squad members will need to merge PRs after they've been approved.

Acceptance Criteria

• Docs are updated
• The peer reviewers group in Silverstripe org permissions are updated if necessary
• The project board is updated if necessary

PRs

• DOC Update docs about community peer reviewers #553

[GuySartorelli]

Updated the project board so it has a "ready to merge" column instead of a "needs second approval" column.