SoapySDR 0.9 release should have online documentation for all APIs

[Nicholas Corgan]

With the upcoming 0.9 release (spoiler alert!), we have two new APIs (plus more post-0.9). These should be posted up where we have the current documentation (https://pothosware.github.io/SoapySDR/doxygen/), but with a higher-level front page with links to each API. Additionally, out of principle (and to avoid complaints like recently), we should have at least a basic Python Doxygen page.

Moving forward, for APIs in other repos that we officially support, we can put them here too.