[REQUEST] Provide API docs with plain doxygen styling

### Is your feature request related to a problem? Please describe.
Previously OpenVDB API docs on openvdb.org used plain Doxygen styling, but nowadays they are styled to look a bit fancier, and integrate the API documentation parts with rest of the documentation in the table of contents sidebar.

As someone who regularly needs to look up OpenVDB/NanoVDB API's I have found the new styling to be a significant regression, class/namespace overviews are now chopped up alphabetically, it is cumbersome to switch between different views, expanding them will only use the narrow side-bar and push other side-bar content off the screen, no summaries of API functions in the namespace/class index, etc.

I often don't know exactly what I'm looking for or what it is called, and previously I would just scan a namespace to find potential candidates for what I need, this doesn't work anymore with the new styling.

IMO the default doxygen styling has much better usability and also has the advantage of familiarity (many other projects use it). I really couldn't care less about the aesthetics of API documentation so it would be nice if there would at least be the option to switch to the classic Doxygen style.

### Describe the solution you'd like
A link on the documentation landing page and/or from the index to open the API docs with classic Doxygen styling

### Describe alternatives you've considered
I can build the doxygen documentation locally and open it from there, but I would much prefer to use the official up-to-date documentation hosted on openvdb.org

Start at the documentation landing page and the current hosted OpenVDB/NanoVDB API documentation to understand how API documentation is linked and generated. The work is done when users can reach an up-to-date API view with classic Doxygen styling from the landing page or index, without losing access to the existing documentation.

Written by the indexing model from the issue text.