Added IR documentation

docs/ir.md

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+# Intermediate Representation
+
+## Static Single Assignment
+
+The SSA IR follows a simple rule: every value is assigned exactly once.
+
+### Basic Blocks
+
+Every function has a list of basic blocks. Basic blocks store values in the order they appear in the original code.
+
+### Instructions
+
+Instructions are no different from values. The calculation `1 + 2` is represented as 3 values using 2 int constants and 1 binary operation:
+
+```
+t0 = 1
+t1 = 2
+t2 = t0 + t1
+```
+
+All of these are considered values even in cases where the type of the value is `void` such as in procedural function calls with side effects.
+
+### Pointers
+
+There are no IDs or indices, therefore values don't know their position in a basic block.
+
+Values reference other values by including a pointer to them.