Rename and relink "Site Availability Engineering" and "Site Reliability Engineering" at /handbook/product/categories/.

This updates the "Product stages, groups, and categories" page (via _categories.erb and stages.yml) as follows:

• "Site Reliability Engineering" has been renamed "Reliability Engineering" and its links have been fixed.
• "Site Availability Engineering" has been replaced by "Delivery Team".

Note: There is still a "Site Reliability Engineer" job family and there may yet be some stray references on other pages.

Closes #5100 (closed)