Admin USB section should link to official Tails documentation

I updated the section that talks about USBs, DVDs, CDs to clarify what is needed before proceeding with the installation. I also updated the section that talks about setting up the Admin USB with links to instructions on the Tails website. Closes #741.
Might as well explain this a little futher, e.g. "the Document Interface runs on port 8080, so remember to append ':8080' to the .onion address when you type it into the address bar".