Add Specializations

[jason-fox]

• Add DTD descriptors
• Amend XSL
• Update documentation

Currently "old style" sample and "new style" sample-ext are both available, but this could be amended.

[infotexture]

Currently "old style" sample and "new style" sample-ext are both available, but this could be amended.

I see the new source files here, but it looks like the new sample-ext versions aren't included in the default .github/dita-ot/html.xml build, so I had to build the docs from sample-ext to see those results.

Apart from updating the code samples to show the specializations instead of the output classes, I think we'd need to explain that there are 2 ways of doing things with this new approach (at least if we plan to include both in a single plug-in).

Per discussion in https://github.com/infotexture/dita-bootstrap/issues/289#issuecomment-4230164369, we might want to consider separating the specializations and shipping them as a separate plug-in, so people can decide which approach they prefer, and keep the docs focused on one approach or another.

The docs for the original version could of course link to the specialized plug-in (and vice-versa), so everyone is aware that both exist.