Add sections algebra and analysis with initial intro files

docs/mathematik/algebra/_category_.json

@@ -0,0 +1,4 @@
+{
+  "position": 2,
+  "label": "Algebra"
+}

docs/mathematik/algebra/algebra.md

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+---
+title: Algebra
+tags: [mathematik, mengenlehre, algebra]
+sidebar_position: 1
+---
+
+WIP