[OE-core] [PATCH 2/2] Add api-doc class

[Zongchun YU]

>>As this class is specific to doxygen, please call it doxygen.bbclass.
[Zongchun] I agree and will change it in the next patch.
>>Also I think this should be opt-in not forcibly enabled as not everyone would want API documentation to be generated.
[Zongchun] Yes. it is opt-in. the source code of packages should be written follow the rules of doxygen. If not. doxygen will not generate docs.
>>Pretty sure you'll want to run doxygen after do_compile in case the documentation is generated from files built at compile time.  But don't most packages that use doxygen invoke it themselves?  You're also not actually installing the documentation anywhere.
[Zongchun] doxygen just use source code(follow the rules of doxygen) of packages to generate docs. not use files built at compile time. Some packages may just need to fetch and patch the source code and then generate docs. the configure file of doxygen have options to specify the generated docs types and location. Whether install the docs may depend the packages.