Split DB Access section into two

[avaczi]

Description of changes:

• Split Managing PostgreSQL Database Access section
  Remove bogus reference to pg_hba.conf, move setting privileges to their own section
• Add references to other documentation sources

An open question remains: is the assumption true that only md5 is available as authentication method?

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[freechaamzn]

Thank you for making this suggestion. I will review it with the PostgreSQL team and let you know if I have any questions.