docs: add user.cfg.5 manual page

docs/config/user/format.rst

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+This file contains the list of API users and API tokens.
+
+Each user configuration section starts with a header ``user: <name>``,
+followed by the user configuration options.
+
+API token configuration starts with a header ``token:
+<userid!token_name>``, followed by the token configuration. The data
+used to authenticate tokens is stored in a separate file
+(``token.shadow``).
+
+
+::
+
+  user: root@pam
+	comment Superuser
+	email test@example.local
+	...
+
+  token: root@pam!token1
+	comment API test token
+	enable true
+	expire 0
+
+  user: ...
+
+
+You can use the ``proxmox-backup-manager user`` command to manipulate
+this file.

docs/config/user/man5.rst

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+==========================
+user.cfg
+==========================
+
+.. include:: ../../epilog.rst
+
+-------------------------------------------------------------
+User Configuration
+-------------------------------------------------------------
+
+:Author: |AUTHOR|
+:Version: Version |VERSION|
+:Manual section: 5
+
+Description
+===========
+
+The file /etc/proxmox-backup/user.cfg is a configuration file for Proxmox
+Backup Server. It contains the user configuration.
+
+File Format
+===========
+
+.. include:: format.rst
+
+
+Options
+=======
+
+.. include:: config.rst
+
+
+.. include:: ../../pbs-copyright.rst