docs: drop the v3 design notes and scenarios from the working branch

All four documents are carried by oras-project#1107, which is where the documentation-only
changes are being reviewed and pruned. Keeping a second copy here only invites
the two to drift, and the copies had already started to: the version of the
config/credentials note on this branch marks the design implemented, while
oras-project#1107 carries the proposal wording that is accurate against main today.

That status update is not lost. It belongs with the refactor it describes and
is recorded on oras-project#1331, to be applied once that lands.

docs/ is now identical to main, and this branch is code only.

Signed-off-by: Terry Howe <terrylhowe@gmail.com>

Author: TerryHowe