games: added mtg(a)

wiki/games/magic:_the_gathering.md

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+# Magic: The Gathering
+
+[Magic: The Gathering](https://magic.wizards.com) - often shortened to MTG or Magic - is a tabletop
+collectible card game.
+There is an online version called [Magic: The Gathering Arena](https://magic.wizards.com/mtgarena) -
+or MTGA for short - that has the same rules as the tabletop variant.
+
+## Resources
+
+There are many useful resources.
+The following list provides a select few of them.
+
+- [deckstats.net](https://deckstats.net/) gives the option to build and analyze MTG(A) decks.
+- [MTG Desktop Companion](https://www.mtgcompanion.org/) is a collection manager for MTG that
+  includes prices, stats and a deck builder, aswell as the option to import and export to most of
+  the used MTG deck formats.
+  It is free and open-source.
+  Its source code can be looked at on their
+  [GitHub Page](https://github.com/nicho92/MtgDesktopCompanion).