Move `design/*` to `designs/*`.

[iphydf]

It's multiple.. also, designs.md shouldn't be called design.md, because then it might be interpreted as the one single toktok design.

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This change is 

[robinlinden]

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Reviewed 7 of 7 files at r1. Review status: 4 of 5 files reviewed at latest revision, all discussions resolved.

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Comments from Reviewable

[robinlinden]

Reviewed 1 of 1 files at r2. Review status: all files reviewed at latest revision, all discussions resolved.

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Comments from Reviewable

[Zer0-One]

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Reviewed 6 of 7 files at r1, 1 of 1 files at r2. Review status: all files reviewed at latest revision, all discussions resolved.

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Comments from Reviewable