rar: added rar package and algorithm

wiki/linux/rar_(package).md

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+# RAR (package)
+
+RAR is a package available for many Linux distributions that handles `.rar`
+archives of the [RAR algorithm](/wiki/rar_(algorithm).md).
+
+## Setup
+
+On most distributions the functionalities for creating a `.rar` archive is
+bundled in the `rar` package.
+For extraction a separate package is used called `unrar`.
+
+## Usage
+
+### Extracting a `.rar` archive
+
+A `.rar` archive can be extracted using the `unrar` package with the following
+command.
+
+```sh
+unrar e <path to .rar file>
+```

wiki/rar_(algorithm).md

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+# RAR (algorithm)
+
+This article describes the [RAR](https://www.rarlab.com/) algorithm.
+For the linux `rar` or `unrar` package see the
+[rar package entry](/wiki/linux/rar_(package).md).
+
+RAR - short for Roshal Archive - is an algorithm for compression aswell as a
+format (`.rar`).
+RAR archives can feature files with extensions of form `.r01`, `.r02`, etc.
+These are parts of the compressed data that are created when the data is
+compressed.