docs: add installation chapter

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>

docs/index.rst

@@ -21,6 +21,7 @@ in the section entitled "GNU Free Documentation License".
    :caption: Table of Contents
 
    introduction.rst
+   installation.rst
    offline-keys.rst
    offline-mirror.rst
    offline-media.rst

docs/installation.rst

@@ -0,0 +1,112 @@
+Installation
+============
+
+Proxmox Offline Mirror package is released as standard Debian package and shipped by the Proxmox VE,
+Proxmox Backup Server and Proxmox Mail Gateway package repositories.
+
+System Requirements
+-------------------
+
+* CPU: 64bit (*x86-64* or *AMD64*), ideally 2+ Cores
+
+* Debian based system (physical host, VM or container)
+
+* Sufficient storage space for the local mirrors.
+  For a basic Debian and Proxmox VE we recommend at least 150 GB
+
+* A filesystem supporting hard links for both, local host and external medias.
+  Note that most Linux derived file systems support hard links, but Windows derived ones (e.g.,
+  \*FAT) do *not* support hard links.
+
+.. _apt_install_pom:
+
+Installation via APT
+--------------------
+
+If the host you want to install the ``proxmox-offline-mirror`` tools already has a package
+repository from a Proxmox Project configured, you can simply install the offline mirror tool through
+``apt``:
+
+.. code-block:: console
+
+     # apt update
+     # apt install proxmox-offline-mirror
+
+If you do not have any Proxmox project's repository set up, see :ref:`package_repos_secure_apt` and
+:ref:`package_repositories_client_only_apt` for how to do so before using above commands.
+
+Debian Package Repositories
+^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+All Debian based systems use APT_ as a package management tool. The lists of repositories are
+defined in ``/etc/apt/sources.list`` and the ``.list`` files found in the ``/etc/apt/sources.d/``
+directory. Updates can be installed directly with the ``apt`` command line tool, or via the GUI.
+
+APT_ ``sources.list`` files list one package repository per line, with the most preferred source
+listed first. Empty lines are ignored and a ``#`` character anywhere on a line marks the remainder
+of that line as a comment. The information available from the configured sources is acquired by
+``apt update``.
+
+.. _package_repos_secure_apt:
+
+SecureApt
+^^^^^^^^^
+
+The `Release` files in the repositories are signed with GnuPG. APT is using these signatures to
+verify that all packages are from a trusted source.
+
+.. tip:: If you install Proxmox Offline Mirror on a system with an existing Proxmox Project's
+  installation, the verification key is already installed.
+
+If you install Proxmox Backup Server on top of Debian Bullseye, download and install the key with
+the following commands:
+
+.. code-block:: console
+
+ # wget https://enterprise.proxmox.com/debian/proxmox-release-bullseye.gpg \
+   -O /etc/apt/trusted.gpg.d/proxmox-release-bullseye.gpg
+
+Verify the SHA512 checksum afterwards with the expected output below:
+
+.. code-block:: console
+
+ # sha512sum /etc/apt/trusted.gpg.d/proxmox-release-bullseye.gpg
+ 7fb03ec8a1675723d2853b84aa4fdb49a46a3bb72b9951361488bfd19b29aab0a789a4f8c7406e71a69aabbc727c936d3549731c4659ffa1a08f44db8fdcebfa  /etc/apt/trusted.gpg.d/proxmox-release-bullseye.gpg
+
+and the md5sum, with the expected output below:
+
+.. code-block:: console
+
+ # md5sum /etc/apt/trusted.gpg.d/proxmox-release-bullseye.gpg
+ bcc35c7173e0845c0d6ad6470b70f50e /etc/apt/trusted.gpg.d/proxmox-release-bullseye.gpg
+
+.. _package_repositories_client_only_apt:
+
+Reuse a Proxmox Repository for Offline Mirror Tool
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+For modern Linux distributions using `apt` as package manager, like all Debian and Ubuntu Derivative
+do, you may be able to use the APT-based repository.
+
+In order to configure this repository you need to first :ref:`setup the Proxmox release key
+<package_repos_secure_apt>`. After that, add the repository URL to the APT sources lists.
+
+We recommend re-using the ``pbs-client`` repository for installing the Proxmox Offline Mirror on a
+vanilla Debian system.
+
+.. hint:: While you could also use a Proxmox VE, Proxmox Backup Server or Proxmox Mail Gateway
+  repository, those ship also some updated packages form Debian native packages, which would get
+  pulled in even if not required for the offline mirroring.
+
+Repositories for Debian 11 (Bullseye) based releases
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Edit the file ``/etc/apt/sources.list.d/pbs-client.list`` and add the following snipped
+
+.. code-block:: sources.list
+  :caption: File: ``/etc/apt/sources.list.d/pbs-client.list``
+
+  deb http://download.proxmox.com/debian/pbs-client bullseye main
+
+Now you should be able to install the ``proxmox-offline-mirror`` package, see
+:ref:`apt_install_pom`.