docs: Make first section be "Get Started" and move the registry content there

[danilo-leal]

Some changes I'm proposing to the docs to mostly make the registry more visible:

• Change the first section from "Overview" to "Get Started"; I think this creates a better segway to present the Registry
  • Includes redirects so that anywhere pointing to /overview/{page} gets sent to /get-started/{page}
  • Includes an update to the page generation script so it points to the new right directory
• Restructure content in the registry page by adding an "Overview" heading, to quickly display information that was below the fold but very relevant to anyone parsing out what the registry is
• Removed agent description from the registry cards; just felt like they were not adding value and cluttering the page