Add extensive documentation for the mittwald CLI

[martin-helmich]

• Add dedicated landing page for CLI
• Link to CLI LP in navbar
• Configure sidebar for CLI section
• Move CLI doc from API section to its own section
• Add redirect rules for CLI pages
• Fix styling of CLI landing page
• Add CLI features on LP
• Also add generator script for importing CLI docs
• Exclude build directory
• Finalize CLI landing page
• Add examples to sidebar
• Download cli directory index as "application/vnd.github.object+json"
• Skip non-md files when importing CLI docs
• Formatting
• Synchronize CLI docs (once)
• Add CLI doc synchronization to regular update job