doc: drop legacy documentation (WIP)

TODO: polish commit message

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  - Remove outdated ATTACKS.md
    requires basic_client.py which was renamed to client.py in 2018
    and is removed in theupdateframework#1790 (host of problems with client.py described in theupdateframework#881)

    --> reference ticket TODO: create ticket

  - QUICKSTART.md also based on cli tools (see above)
    --> reference ticket TODO: create ticket

Signed-off-by: Lukas Puehringer <lukas.puehringer@nyu.edu>