Comprehensive user guide rewrites/updates

[ntoll]

DO NOT MERGE this is a work in progress.

This PR is stacked on the work done to automatically generate API docs from our source code. It must only be merged after the next release of PyScript, since it documents and makes use of some of the breaking (pyscript.web related) changes.

This PR involves:

• Restructuring the documentation into more logical sections.
• Simplifying the structure so duplications and complex cross-references are (to the best of our ability) removed.
• Checking the content for accuracy.
• Updating the language to make it simpler, better organised and easier to understand.
• Adding example apps based on specific PyScript features.
• Removal of the old PSDC based example apps.
• In the user guide, always having a "what's next" section at the end of a page so folks have somewhere to go.
• In the user guide, embedding the example apps in appropriate places.

TODO:

• Check again the example apps.
• Ensure example app information is coherent.
• Check FAQ.

FEEDBACK WELCOME