Add polished v1 (rc-1) version of log.md

The updated log.md is meant to be a dense specification, mainly to be read by implementers and other readers that already have a grasp of how a transparency log works. A separate document should explain why we arrived at this specification. Similarly, separate documentation should outline how to use the sigsum system (overall utility, tooling, etc).

Summary of changes:

• Add status and title to communicate this is a release candidate.
• Add abstract, table of contents, and how to cite log.md. To make the sections look nicer, also replaced "1.1 - Foo" with "1.1. Foo".
• Minor edits to introduction, including adding the missing figure.
• Cut old overview in favor of the already added introduction. (Most of this could be reused when adding the new Section 2.2.1 though.)
• Ensure that Section 2 is easy to refer to, e.g., from the companion witness document that uses the same HTTP request-response format.
• Overall, a lot of clean-up in Section 2 to "just state the facts".
• Put all status codes into a table for overview (start of Section 3).
• Add missing and up-to-date examples in Section 3.
• Cut the old Section 4, "Summary of parameters". Doesn't add much.
• Consistent indent (width 72), punctuation, etc.

Closes #11 (closed).