Documentation quality guidelines for the PostgreSQL team

As part of our newly agreed team responsibilities and workflow https://www.notion.so/percona/Team-Responsibilities-and-Workflow-Alignment-18b674d091f38067ac88f3c287a1dc0a?pvs=4

One task is for @Anastasia Alexandrova to write done documentation and quality guidelines for the team to follow.

Definition of Done:

@Anastasia Alexandrova wrote down quality guidelines for documentation, that are understood and can be used by the overall PostgreSQL team.