Convert the CATS documents from AsciiDoc to Markdown

[harrdou]

The current use of AsciiDoc and the manner in which the SAML specification document is dynamically built by merging Kantara and GC sources was a labour saver in 2019/20 when both the Kantara and CATS specifications were actively under development.

Now that the Kantara spec has stabilized, it makes more sense to build a static document, so that it is easier for stakeholders to contribute improvements in a more traditional way (through pull requests). Moving from AsciiDoc to Markdown will also serve to make the spec more contributer friendly, while also allowing the application of the existing Jeckyll tooling for the GCWeb theme.