Support namespaces rendition for class-like structures

[Girgias]

Examples:

• BcMath\Number
• Pcntl\QosClass

Related to php/doc-en#4516

[haszi]

Are we looking for something like this:

namespace BcMath;

final readonly class Number implements Stringable {
...

Would we be using DocBook elements to annotate namespaces or just split classnames by the \ character?

I was trying to find a suitable DocBook element but nothing seems to be a really good match. On the other hand, I see that we already used <package> once to refer to one of the extensions / namespaces in doc-en.