Update links to EPUB specification to EPUB 3.3 spec

## Description
The XML documentation for the EPUB schema classes references the EPUB 2.0.1 and EPUB 3.2 specifications. EPUB 3.2 specification is now outdated and has been replaced with EPUB 3.3 specification.

## Proposed solution
1. Change the links in the documentation to the EPUB 3.3 specification.
2. Update the issue templates with the new links.