Improve the documentation

[pyrech]

The documentation have been reworked a bit:

• Smaller main readme with a "Quick start" chapter
• Move documentation to /docs directory to match GitHub standards
• Better structure and navigation in the documentation
• Add documentation about iteration / pagination
• Add a GitHub pages for the whole documentation with Docsify:
  • only a small index.html is commited in docs/ folder to configure Docsify
  • nothing to build - it automatically transforms existing Markdown file from the main branch in HTML page

You can see the final result here: https://jolicode.github.io/slack-php-api/