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.
`user_manual.md` no longer exists, it was split into separate manuals for sources and journalists. This should point to `journalist_user_manual.md`.