[docs] Add link to documentation

[mottosso]

Noticed some documentation in the docs/ folder, and searched the issue section for clues as to where it may be hosted.

Let's make it official!

• https://nanogui.readthedocs.io

[wjakob]

There is actually a button, but this may be a little subtle. Adding an extra link sounds like a good idea.

[mottosso]

Ah so there is, did not notice it until now. I was also text-searching for "docs" and "readthedocs" but came up empty.