test_generators: Replace CUR_DIR with CONTENT_DIR for subdir cat. detection

In situations where I've cleared ARTICLE_DIR, I've done so to ensure
that there are no directories that will override the DEFAULT_CATEGORY
due to USE_FOLDER_AS_CATEGORY.
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
generators: get_files() should use paths relative to Generator.path

All paths should be relative to Generator.path unless we're actively
accessing the filesystem.  This makes the argument less ambiguous, so
we have less likelyhood of joining paths multiple times.