Create index_de.html

A German translation of the central page. From my point of view, this is enough for a first internationalised version although the about page is still english only.

Notes.

# I changed the logic of the major heading to {{if ...}}
# changed the Wikipedia links where applicable
# left the <title> but changed the navigation entry - maybe reverse that, to keep the branding
# reasons for weak ciphers are still english (located in ../insecure_suites.go, see comment "by ju")
# I added the possibility that keys are aquired later in the argument for Forward Secrecy (see comment "by ju")