generators: Update PagesGenerator to use new read_file

Also standardize signal names.  If `article_generator_*` is singular,
`page_generator_*` should be as well.  Fix it from the older
`pages_generator_*`.
generators: Convert StaticGenerator to use the new read_file
Remove the FILES_TO_COPY setting

We no longer instantiate the Static object in the StaticGenerator, so
we can't set the save_as argument anymore.  If you want to adjust the
output path, use the upcoming EXTRA_PATH_METADATA setting.
Add the EXTRA_PATH_METADATA setting

Useful for altering static output paths when you don't want to encode
extra metadata in the static file's source path.
Update sample configurations from FILES_TO_COPY to EXTRA_PATH_METADATA
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