Docs: Add KTX2Exporter page

[mkeblx]

As title says.

[Mugen87]

Notice that the module is already documented as JSDoc and there is actually a template in place that generates a new documentation. Try the npm script npm run build-docs and the open it locally via http://localhost:8080/docs_new/.

Unfortunately, we had not the chance yet to move it to production since there are some open issues with the template style and redirects. However, before you invest more time in the "old" docs I think it's important to state that the JSDoc is the primary documentation now.