Generate documentation

[htdvisser]

We currently have the Frequency Plans Overview and Frequencies By Country pages (as well as a map) on our documentation site.

Now that our frequency plans are kept in a more structured format, those documentation pages may be generated and updated automatically when things change in here.

[htdvisser]

Pushing this to Q2. Would be much easier to do when we have the Go code of pkg/band and pkg/frequencyplans from TTS in here (see also #38).