Expanded test_docs significantly

* Added docs section to yaml template to define docs which should be
present on an appliance
* Updated 'requirements' to include 'PyPDF2'
* Added docs_info fixture to pull from yaml
* Renamed test_docs() method to test_docs_links()
* Added test_docs_info() method to check that links on page all conform
to the same standards, ie have same text, matching urls
* Added test_docs_contents() method to download PDF to memory and
confirm the document title matches what is expected from the link to
some degree
* Added test_all_docs_present() method to compare against yaml to see if
all the required documents are present on the page