[osmocom-analog] Osmo-CC documentation

Hi Andreas, this is really very helpful, thank you so much. I was able to understand how some of the options work from looking at the source code, but I still feel I lack some basic knowledge of how the different Osmo-CC components interact, and this documentation should help close those gaps quickly. For instance, using osmo-cc-sip-endpoint with a SipGate account and osmocom-analog's C-Netz module, I was able to make incoming calls from the PSTN to mobile C-Netz devices registered against osmocom-analog. I was not able to make outgoing calls, though, as I seem to be unable to make either osmocom-analog understand it needs to route the outbound call to osmo-cc-sip-endpoint, or make osmo-cc-sip-endpoint understand it must seize the call attempt and connect the call through the SIP provider. Also, I was unable to make the speaking-clock Zeitansage example work with osmocom-analog so I could call 1191 from the C-Netz mobiles registered against osmocom-analog, and I assume this is due to my lack of understanding on how to ensure calls to 1191 are properly routed from osmocom-analog to the Zeitansage module. Since the Zeitansage, in effect, is just another endpoint, I should be able to gain helpful insight from the documentation in that respect as well. Also, the OsmoCC-Router looks quite powerful and should allow for very flexible routing between a multitude of endpoints, based on routing scripts, so a documentation on this, as foreseen here for future additions, will also be extremely helpful. Thank you very much for this work. BR, Markus