Optimize Readme documentation

[hagenburger]

• Fix typos
• Check variable/command names
• Change order in code examples:
  1. Imports
  2. Definitions, configuration, plugins
  3. Markdown code
  4. Rendering
• Add PIMD instructions for highlights/showmore
• Add setup instructions for plugins
• Add missing documentation
• Fix existing regular expressings (mostly escaping issues)
• Add link to Prism plugin

To review this, I put all README.md rendered with PIMD online here: https://hagenburger.github.io/pimd-docs/ (please follow the links in the “Plugins” section to see all documents)