Updates to Docs

[lesamouraipourpre]

• Correct the links to PyPI and readthedocs (hyphens not underscores)
• Add cross-reference links in conf.py to adafruit_displayio_layout, specifically for the Widget and Control class.
• Extract the guide from displayio_switchround.py to docs/overview.rst. This dramatically reduces the size of displayio_switchround.py for anyone installing the .py file
• Moved the Inheritance diagram from the bottom of displayio_switchround.py to near the top.
• Add the Inheritance diagram to the overview.rst guide at the place discussing inheritance.

@kmatch98 I've tried to make only minimal changes to the wonderful work you've done on this, focusing mainly on adding cross-reference links. Can you have a read of this to confirm that it still makes sense, please?

@jposada202020 Does this separation of the overview.rst make sense with what you are trying to achieve in library documentation?

[jposada202020]

@lesamouraipourpre Indeed as you have witnessed my efforts to reduce memory footprint, this makes sense for boards with low memory avalaible, as the heap size limits the amount of code lines they can load.
Also this is line to our design guide, where we recommend to document in the library the parameters and the simple usage.

[jposada202020]

and a +1 for @kmatch98 documentation, it was the source for our standard on how to use code in the libraries and in our design guide. 👍🏼

[kmatch98]

Seems like a great idea. I didn’t build or review but feel free to reorganize. I especially appreciate taking the docs out of the code file since it will shrink the file a bit.

[kattni]

I think this is fine. As long as the docs end up generated, it doesn't matter from where they're generating. Thanks for the work on this!

[lesamouraipourpre]

Good catch @jposada202020. I think I had gone document blind by that point - If it drew a diagram it must be correct.

[jposada202020]

THANKS!! @lesamouraipourpre