Add policies for global attribute conflict resolution upon attach

For example when loading parent_merge.xml:

```
WARNING: Attach conflict when attaching 'child' to 'parent_merge', policy is 'merge'
timestep: parent has 0.005, child has 0.002, taking the minimum
iterations: parent has 50, child has 100, taking the maximum
flag 'Damper': added from child
```

When loading parent_error.xml:

```
XML Error: Attach conflict when attaching 'child' to 'parent_error', policy is 'error'
timestep: parent has 0.005, child has 0.002
iterations: parent has 50, child has 100
Element 'attach', line 10
```
PiperOrigin-RevId: 933620810
Change-Id: Ib477863b5ef763474d27fb4be5a4148be1d5d500

doc/XMLschema.rst

@@ -254,6 +254,9 @@
          .. grid-item::
             :ref:`alignfree<compiler-alignfree>`
 
+         .. grid-item::
+            :ref:`conflict<compiler-conflict>`
+
 
       .. dropdown:: :ref:`lengthrange<compiler-lengthrange>` :octicon:`dot`