Update docs.nethserver.org landing page

[DavidePrincipi]

URL https://docs.nethserver.org now shows just the NS7 admin's manual.

• Links to latest stable and old stable releases of NethServer must be shown instead
• Show also links to NethSecurity project
• NS7 admin's manual must display a warning that NS8 is now the latest stable release
• Evaluate ads-free offerings of ReadTheDocs hosting

Discussion https://mattermost.nethesis.it/nethesis/pl/o5ga4cyynfy6u8ns4xuot74wch

[DavidePrincipi]

The landing page will be part of a major docs.nethserver.org rework, following the same path of docs.nethvoice.com.