Adapt README sections for website

[feloy]

Now that README.md is used to build a multi-pages website (https://kortex-hub.github.io/kortex-cli/), some modifications are needed to make the navigation better:

• home page is empty, let's move the content of the Introduction section to the home page
• Runtimes contains only one section: Podman Runtime. Let's move the Podman Runtime section as a top-level section, and we will place other runtimes at the top, as these are important information
• add a badge linking to the github pages documentation
• increase the level of subsections displayed in the table of contents on the right side of the page (from 1 to 2)