Move psa-thread-safety.md

Signed-off-by: Ryan Everett <ryan.everett@arm.com>
diff --git a/docs/architecture/psa-thread-safety.md b/docs/architecture/psa-thread-safety/psa-thread-safety.md
similarity index 100%
rename from docs/architecture/psa-thread-safety.md
rename to docs/architecture/psa-thread-safety/psa-thread-safety.md