app-doc/cantera-docs: New package

app-doc/cantera-docs: New package

The commit adds new package 'app-doc/cantera-docs' (2.4.0) that install
the API documentatoin for 'sci-libs/cantera' package.

Description:
Cantera Doxygen API Documentation for C++ library and Sphinx API Documentation
for CTI tool, Python module and Matlab interface.

Unfortunately there is no official documentation tarball for latest version
of Cantera. So this package download the tarball build by myself from sources.
This tarball includes modifications of 'Menubar' to navigate local sections.

https://user-images.githubusercontent.com/18756734/46048699-7bd74b00-c133-11e8-8fe2-77480a33ef19.png

and patch for async loading remote javascript and css files that allow
to render pages without long delays in some cases of offline reading.

Signed-off-by: Sergey Torokhov <torokhov_s_a@mail.ru>
Closes: https://github.com/gentoo/gentoo/pull/10017
Signed-off-by: Michał Górny <mgorny@gentoo.org>