devel/cl-alexandria: Split the PDF documentation into its own option.

Building the PDF needs texi2pdf and pulls in TeX, which is a lot of
machinery for a small utility library, so it now lives behind a PDF
option that is off by default. The HTML and info files only need
makeinfo and are still built with DOCS.

Also install alexandria.info: DOCS_INFO pointed at doc/alexandria, but
nothing ever installed the file, so the port registered an info entry
it did not ship.