Add new translation language: Galician (#613)

* Added Galician translation (incl. some translated images, mindmap, and pattern template)
* Added @psanxiao as translation lead for Galician

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Co-authored-by: Sebastian Spier <github@spier.hu>

Author: psanxiao

.github/CODEOWNERS

@@ -16,3 +16,5 @@
 /book/zh/           @WillemJiang
 /translation/pt/    @jrcosta @zilio
 /book/pt/           @jrcosta @zilio
+/translation/gl/    @psanxiao
+/book/gl/           @psanxiao

.github/workflows/book.yml

@@ -15,7 +15,7 @@ jobs:
 
     strategy:
       matrix:
-        language: [en, ja, zh, pt-br]
+        language: [en, ja, zh, pt-br, gl]
 
     steps:
     - uses: actions/checkout@v3