test_readers: Update to new readers.read_file
test_generators: Replace CUR_DIR with CONTENT_DIR for subdir cat. detection
contents: Page fallbacks for context and localsiteurl
content: Don't update static content
settings: Add a warning for folks using FILES_TO_COPY.
readers: Ensure the reader class is enabled before instantiating

Otherwise the MarkdownReader fails with:

  'bool' object is not callable

if Markdown is not installed.

Reported-by: Deniz Turgut <dturgut@gmail.com>
docs/plugins.rst: Document signal name changes
Document path metadata extraction