Various updates to the docs

Since I started contributing to PyScript, I have made a lot of updates and improvements to the official documentation.

So far these are the changes the docs:

- Improved the get started guide
- Added more entries in the Reference guide
  - Added docs for Element API
  - Added docs for py-terminal
  - Added docs for exceptions and error codes
- Added tutorial on how to use fetch from py-config
- Added tutorial on writing content to the page