[Docs]: Update and improve quickstart

[szaganek]

1. Combine repeated content into one, helpful topic in CLI docs:

   • https://docs.apify.com/cli/docs/quick-start
   • https://docs.apify.com/platform/actors/development/quick-start/locally

2. Adjust it to the style guide.

3. Update the overview topic so the links are correct:

   • https://docs.apify.com/platform/actors/development/quick-start