asciidoctor-pdf: convert to new spec format

[ptomsich]

This converts the document into the new RVI documentation style for ISA extensions in PDF form (as used for public review):

• adds a .gitignore
• adds a Makefile that generates a PDF
• adds the template and resources
• converts Zicbom/Zicboz into the new format
• adds Wavedrom instruction/opcode encodings

Note that some content will still have to be adjusted by DavidK.

[dkruckemyer-ventana]

I'll merge this for now to test out on my local system, but the actual content still remains in the /specifications directory