Complete documentation for UEFI boot

Our initial UEFI support is part of Tails 1.1. The bare minimum of documentation is updated (#7220 (closed)), but there will likely be more to check, update and improve:

• adapt our existing documentation to take into account that UEFI boot is now possible:
  • known issues page
  • booting Tails page (?)
  • about and/or features
  • hardware requirements
  • Mac-specific doc: we need to keep the existing manual installation documentation (as part of solving the bootstrap problem for those who have neither a PC, nor a DVD writer, nor a trusted Tails device); but, we must also document that a device installed by Tails Installer boots fine on most Macs provided rEFInd is installed
• document known issues of our UEFI boot support, and corresponding workarounds when applicable; these are gathered on the blueprint, and more specifically:
  • non-goals section
  • the test results section mentions a few hardware-specific problems and tricks

Related issues

• Blocked by #7230 (closed)

Original created by @intrigeri on 6562 (Redmine)