📝 add adr about iterator facade

docs/adr/0002-iterator-facade.md

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+# 2. Iterator Facade
+
+Date: 2023-10-06
+
+## Status
+
+Accepted
+
+## Context
+
+Writing stl complain iterators is a troublesome adventure. The introduction of 
+a facade will minimize the needed effort to write iterators and would avoid 
+hard-to-detect errors in the long run. A variety of implementations for such a 
+facade are already available:
+
+* [Proposal: `std::iterator_interface`](https://wg21.link/p2727r1)
+* [Code: `boost::iterator_interface`](https://github.com/boostorg/stl_interfaces)
+* [Code: `boost::iterator_facade`](https://github.com/boostorg/iterator)
+* [Blog: An `iterator_facade` in C++20](https://vector-of-bool.github.io/2020/06/13/cpp20-iter-facade.html)
+
+## Decision
+
+Introduce an iterator facade before implementing any containers to reduce the 
+implementation and maintenance effort in the long run.
+
+## Consequences
+
+Using an iterator facade would create a dependency between most 
+iterators and algorithms. Also, the compile time will suffer 
+because of the additional template instances needed.