Documentation - Fix a bunch of typos, and improve certain parts of the documentation

[csd4ni3l]

I went through almost the whole documentation, and fixed some typos, and rephrased in some places. This is my first contribution, so if i did anything wrong, please correct me.
Files modified:

- opengl_notes.rst
- arcade_book.rst
- install.rst
- own_layout.rst and own_widgets.rst
- sprites/advanced.rst