Add a Sphinx role for linking CVEs to the ISC Knowledgebase

Add a Sphinx role for linking CVEs to the ISC Knowledgebase

The new :cve: Sphinx role takes a CVE number as an argument and creates
a hyperlink to the relevant ISC Knowledgebase document that might have
more up-to-date or verbose information than the relevant release note.
This makes reaching ISC Knowledgebase pages directly from the release
notes easier.

Make all CVE references in the release notes use the new Sphinx role.