Continues the Bookshelf history. The three plugins tracked there are
consolidated into umbra-bookshelf/ and this repository becomes the canonical
home for the plugin and the theme.
bd-bookshelf/ -> theme/ separate artifact, name unchanged
(root specs) -> umbra-bookshelf/spec/ aligned to Umbra Bookshelf
old plugin zips -> archive/pre-umbra-releases/
Adds a repository README and drops the Bookshelf closure notice.
