Improve documentation for the Mat4 projection functions

[kovaxis]

Like I mentioned in #569, this PR adds more detailed documentation to the Mat4::perspective_* and Mat4::orthographic_* family of functions, as well as adding Mat4::perspective_rh_vk and Mat4::orthographic_rh_vk.

I initially wrote a very detailed doc explaining what world-view coordinates get mapped to what normalized-device-coordinates, but they look very out-of-place and are probably too much. Instead, I added a small subtext among the lines of Useful to map the standard right-handed coordinate system into what OpenGL expects..

I haven't implemented the *_rh_vk functions or their tests yet. I would like to receive confirmation that this PR is ok before doing so.

[bitshifter]

Hi, because there are 8 different projection methods currently adding a vk suffix will double that. At some point I'd like to revisit how this is done in glam, probably separating these out into separate modules for handedness and graphics api so users only get one in their project, although realistically I'm not sure when I will get around to that.

Happy to accept the documentation improvements.