Update PELD document against the future scope of Tails

Originally created by @Anonymous on #11777 (Redmine)

During the summit this year, we talked about the future scope of Tails, which is documented in the summit repo.
The PELD (https://tails.boum.org/contribute/design/) should be gone through and checked & updated against what we defined there.

Related issues

Related to #11778 (closed)

Edited May 15, 2020 by Anonymous

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