Absorb the matrix-react-sdk repository (#28192)

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docs/features/README.md

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+# Feature documention
+
+The idea of this folder is to document the features we support in different parts of the app.
+In case anyone needs to work on a given part, and isn't aware of all the features in the area,
+they will hopefully get an idea for all the supported functionality to know what to take into account
+when making changes.