pw_cpu_exception: Add CPU exception module

This change introduces pw_cpu_exception, a facade that provides a
high-level interface for handling CPU exceptions.

Change-Id: I1f98ac82dd54040448b2e2ac32a392c49cb6dfba

docs/BUILD.gn

diff --git a/docs/BUILD.gn b/docs/BUILD.gn
index 2133e55..c158e8a 100644
--- a/docs/BUILD.gn
+++ b/docs/BUILD.gn
@@ -41,6 +41,8 @@
     ":target_docs",
     "$dir_pw_bloat:docs",
     "$dir_pw_build:docs",
+    "$dir_pw_cpu_exception:docs",
+    "$dir_pw_cpu_exception_armv7m:docs",
     "$dir_pw_docgen:docs",
     "$dir_pw_dumb_io:docs",
     "$dir_pw_dumb_io_baremetal_stm32f429:docs",