Minor grammar improvements to documentation

Hi,

I found some spelling/grammar mistakes in the docs so I've made some corrections. Some of it is just phrasing that sounds slightly unnatural to me as a native english speaker. This is my first contribution so I've avoided changing any of the actual content, except for one dead link in the "how to write a tutorial" tutorial which I've just removed.

I noticed there are also Doxygen markdown files on master, which don't exist in 2.4. I assume these will need to be changed as well but it wasn't clear to me how this should be handled while sphinx is still in use.