Add support for generating enum pages for the manual

[kocsismate]

I intentionally omit the usage of the enumitemdescription element, and I generate links instead based on php/phd#180 (comment).

[DanielEScherzer]

I'm not sure I'm qualified to review the doc-generation parts of the gen_stub script, deferring to @Girgias for that
But, should this be targeting master instead of 8.5?

[kocsismate]

but, should this be targeting master instead of 8.5?

No, I intentionally targeted 8.5 because that's the version which is being synchronized with the manual.

[Girgias]

Why do you need this? I don't understand why the enum case needs to know about the name of the enum it's part of?

[kocsismate]

All VariableLike classes do the same (check ClassConstName and PropertyName) because the class/enum name is needed in multiple places:

• ClassConstName and PropertyName have a __toString() method where the classname is needed
• The class/enum name is needed for all these classes to be able to generate the linkend attribute (check the ClassConstName::getFieldSynopsisDefaultLinkend() and the EnumCaseName::getEnumSynopsisLinkend() methods)

That said, the enum name could be stored in the EnumCaseInfo directly instead of EnumCaseName, but I figured it's a good idea to follow the other name classes.