Each shader now has sample image, example mesh configuration and example
rendering setup. Also properly documented all attribute types and made
introductory chapter for whole Shaders namespace.
It seems like a bad idea, but it will:
* Improve portability, as `Int` will be always 32bit.
* Improve readability, as `std::int32_t` is just plain ugly and too
complicated to write.
* Improve consistency and reduce confusion, as it's not good to mix
`int`, `std::int32_t`, `GLint`, `khronos_int_t` and whatnot in one
codebase.
* Possibly reduce compilation time, because including all ~35k lines
worth of GL headers just for one GLfloat typedef is even worse than
now forbidden #include <iostream> in headers.
* Grouping lengthy documentation into "Feature overview" page.
* Link only to example index, where all the disclaimers are.
* Also minor documentation updates.