chore: Improve the README

[gonfunko]

The basics

• I validated my changes

The details

Proposed Changes

This PR makes a number of changes to improve the README. Specifically, it:

• Updates all links to our current websites
• Adds a link to a live demo of Blockly
• Improves the alt text on the screenshot
• Recommends using the create-package script and provides an example
• Adds a section about our accessibility features
• Removes the shoutouts at the end, which I believe are obsolete at this point
• Removes a dead link to the form to tell us about how you're using Blockly

[rachel-fenichel]

LGTM.

One requested follow-up: update the create-package README to start with how to use it to create an application, then move to using it to create a plugin. It's slightly confusing that we link to it for creating an application, but all information above the fold is about creating a plugin.