Add any missing sources for third-party info as footnotes where possible

[rdw-software]

Now supported on GitHub: https://github.blog/changelog/2021-09-30-footnotes-now-supported-in-markdown-fields/

I've had to track down some sources after forgetting where the info is from after all this time, so linking them directly seems like it would avoid this problem. It would make it easier to double-check old info to disprove or verify it once more knowledge is available, which has been required most recently with the GND lightmap slices, for example.