oll-doc: Add link to PDF version of usage example

If a PDF is present in the usage-examples folder, the usage example
section (in file output) starts with a link to the PDF file.
oll-doc (usage example): Determine include file

As the "snippet" and the usage example share the basename it's not
necessary to manually define the include's basename.
oll-doc: Slightly improve detail page CSS
oll-doc: Add initial tagline/copyright to usage ex.
oll-doc: Make LP and HTML output of include file consistent

The "file to include" is now printed below the "Usage" heading
both in the HTML page and the LilyPond generated usage example.
line-break-marks: Update files and docs