Generate accessible PDF

[alexduryee]

At some point, we'll want to generate accessible PDF documents for the TLs via the XML->FOP workflow.

cc @fordmadox

[alexduryee]

To-dos from @fordmadox

• Generating the PDF by invoking FOP's accessibility mode (ref. https://xmlgraphics.apache.org/fop/2.3/accessibility.html)

• Add embedded metadata to the PDF via the XSLT transformation to meet PDF/UA standards

[fordmadox]

@eboudreau , @alexduryee : I think that we might be able to use the 2018 minor EAC revision to create a PDF/UA file. I don't think that there are any images or graphics in the EAC tag library, so it should just be a matter of a few minor changes to create a PDF/UA tag library. Once we get to the PDF creation step, just let me know and I can take a crack at that.

[alexduryee]

Thanks! I generated the EAC-CPF PDF using fop -a, which enables accessibility output (https://svn.apache.org/repos/asf/xmlgraphics/site/deploy/fop/trunk/accessibility.html), and ran Acrobat's Accessibility Checker against it.

To-dos, in order to pass the Accessibility Checker:

• Add a document title in metadata (may as well add other metadata elements while we're at it)
• Add bookmarks within the PDF
• Add alt-text to images (I think it's just the SAA logo and the example element?)
• Add headers to tables. This one is a little vexing, since the list of attributes for each element is stored in a table. We'll need to add "attribute" and "required?" headings to each of those tables.

Once these are done, I think we should be set for generating PDF/UA as part of the process.