Proposal: how to use the doc/proposals folder

Background
---
It is a bit inconsistent to have some proposals in archive and others in
doc/proposals.  This happened because:
	1. Our process is to persist pads in the archive.  Some proposals were pads.
	2. The proposals in doc/proposals were a bit more structured.  It added
	overhead that we felt was not necessary to have yet.

Proposal
---
	1. Move proposals that were persisted in the archive to doc/proposals as is.
	To keep it simple, only move proposals that have "proposal" as part of their
	persisted name.
	2. Update affected links so that they are not broken, should not be many.
	3. In the future we put all types of proposals in doc/proposal, regardless
	of if they are related to the Sigsum protocol or something else (like this
	document).  Notably this includes pads that are to be persisted by the chair
	after our weekly meets.
	4. The main criteria is that a document in doc/proposals proposes something.
	There is no particular format for a proposal's content, other than being
	plaintext viewable.
	5. To keep the listing clean we continue to prefix proposals with
	YYYY-MM-<name>.  The date refers to when the document was initially added.

Notes
---
A notable pro with this is that it is easy to update a proposal if we realize
that something is missing after persisting it.  Our archive is not meant to have
updates.