docs: add Arize AX callout to the top-level Phoenix overview page (#15723)

* docs: add Arize AX callout to top-level Phoenix overview page

* docs: simplify AX callout and add Phoenix vs AX card to Other Resources

* docs: reword AX callout to managed enterprise platform

* docs: drop AX info callout, keep Other Resources card

* docs: promote AX callout to a horizontal card

* docs: present AX callout as paired cards with product and comparison links

* docs: add intro sentence above AX cards