atomics: convert to reStructuredText

No attempts to fix or update the text; these are left for the next
patch in the series.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

docs/devel/index.rst

@@ -17,6 +17,7 @@ Contents:
    loads-stores
    memory
    migration
+   atomics
    stable-process
    testing
    decodetree