Update PELD document against the future scope of Tails

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)

Original created by @u on 11777 (Redmine)

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