Doc generator creates links to features in excluded namespaces

[j8r]

Crystal doc lists all Direct Known Subclasses, even :nodoc: hidden ones. Clinking on them leads to a dead link.

Example: https://crystal-lang.org/api/master/Struct.html#direct-known-subclasses with Crystal::DWARF::Abbrev, and other objects.

[Blacksmoke16]

This seems to be related to :nodoc: not being supported when defined as part of a higher level namespace. I.e. its defined as part of Crystal::DWARF, but not the Abbrev type within that namespace.

[straight-shoota]

#9819 fixed :nodoc: namespaces, but there are other reasons for not being included in the documentation (lib types are not documented). So I've reopened this to keep track of that.

IMO the best solution for this would be a complete refactoring to make the doc generator keep track of which namespaces are included and simply check against that list when generating links etc.

cf/#9931 (comment)

[straight-shoota]

The generator als lists class methods (probably instance methods as well?) inherited from an undocumented ancestor.
For example on https://crystal-lang.org/api/1.13.1/File.html: "Class methods inherited from module Crystal::System::File".

[straight-shoota]

#14878 and #14878 were both intended to fix this but they were insufficient and got reverted. So we need to reopen this again.