Polish a v1 draft of log.md

There are no planned changes to log.md at this time, and only relatively minor changes still ongoing that affect witness.md. This means that it is a good time to start polishing log.md as a v1 draft.

Some notes from a chat with nisse:

• We likely want to retire design.md and make log.md a (dense) standalone document, which probably means moving in (non-)goals and threat model into log.md and possibly a very brief "architecture". The "this is how you would use sigsum at large" (the later parts of design.md) is not in-scope for log.md, which is meant as a reference for implementors and those that already understand tlogs and are looking for the details.
• Revisit documentation of "why was sigsum designed like this" ("FAQ" in design.md, and maybe in other places too) and "how do i use sigsum" separately. The latter we already made a lot of progress on in sigsum-go/docs.
• Refer to witness.md as a buliding block from log.md.
• nisse will start by taking a stab at sketching out some changes, or just take a pass and then ask for feedback (whichever he prefers).