[Docs] Another batch of docs improvements

[peterschmidt85]

• Document retry policy on the tasks concept page and the task reference page
• Move dstack Sky instructions from the installation page to a separate guide
• Remove the run arguments documentation from the task reference page
• Update Quickstart with more simple configuration examples
• Update the basics pages with more real examples
• State explicitly that dstack is a lightweight alternative to K8S (that works with K8S as well)
• Page structure improvements
• Replace dstack run with dstack apply
• Add comments to configuration properties (WIP)
• Llama 3.1 example (WIP)
• TRL example (WIP)