At/ldap

Summary

• connect.md: mention LDAP support and its configuration
• ldap.md: update article

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• Close #429
• Cherry-pick to: none
• Staging: https://docs.binary.picodata.io/picodata/at_ldap/tutorial/ldap/, https://docs.binary.picodata.io/picodata/at_ldap/tutorial/connecting/#pgproto