Add migration guide for #20522 (#20525)

# Objective

#20522 counts as a breaklng change, but didn't have a migration guide.

## Solution

Add the migration guide :D

Author: Jondolf

release-content/migration-guides/rot2_matrix_construction.md

@@ -0,0 +1,25 @@
+---
+title: "Fix `From<Rot2>` implementation for `Mat2`"
+pull_requests: [20522]
+---
+
+Past releases had an incorrect `From<Rot2>` implementation for `Mat2`, constructing a rotation matrix in the following form:
+
+```text
+[  cos, sin ]
+[ -sin, cos ]
+```
+
+This was actually the *inverse* of the rotation matrix, resulting in clockwise rotation when transforming vectors.
+The correct version is the following:
+
+```text
+[ cos, -sin ]
+[ sin,  cos ]
+```
+
+resulting in counterclockwise rotation.
+
+This error has now been fixed. You may see that rotation matrices created using the `From<Rot2>` implementation
+produce different results than before, as the rotation happens counterclockwise (as intended) rather than clockwise.
+Invert either the input `Rot2` or the resulting `Mat2` to get the same results as before.