Include NitroKey 3 in MFA doc

[UndeadDevel]

This PR adds documentation for the recently added NK3 support to the qubes-app-yubikey package. I've also integrated the changes of #1372 (except the Google Authenticator part), because that PR changes the name and structure of the file and it makes sense to adapt my changes to it. Do note that the referenced PR's inserted block from line 28 to 123 should thus be merged also from #1372 to include Google Authenticator instructions.

Regarding NitroKey and pipx: I've waited now for almost two months and their binaries are still not signed, so I've simply linked to their installation instructions and put a prominent warning against pipx in this doc PR. Their main developer did confirm in their forum that detached signatures will be added in one of the next releases, however, and they also have the intention to package pynitrokey eventually, which I also reference in this doc.

Fixes QubesOS/qubes-issues#3307

Edit: checks are not failing due to this PR:

Checking internal link hashes in 196 files
Ran on 1320 files!
For the Links > Internal check, the following failures were found:
* At ./_site/doc/device-handling-security/index.html:424:
  internally linking to /news/2018/09/11/qubes-ctap-proxy/, which does not exist
* At ./_site/news/2018/09/11/qubes-u2f-proxy/index.html:364:
  internally linking to /attachment/doc/u2f.svg, which does not exist
HTML-Proofer found 2 failures!
Some checked failed. See above.
+ all_ok=false

[unman]

@UndeadDevel Can you review this after the merging of #1372?

[UndeadDevel]

Conflicts are resolved; ready to merge.

NitroKey still hasn't started providing signatures for their artifacts smh...it's been >4 months and several pynitrokey versions now since they promised that, but, alas, the doc has a big fat warning and should be robust to change as long as they update their docs, which mine link to, once they finally provide signed artifacts or even proper packaging.

Checks still failing for the same reason as before (see above)...someone should probably fix that.

Oh and I just noticed: I think this can be considered as Fixes QubesOS/qubes-issues#3307