Add explanations and restructure into themes

[emiltin]

We need more verbal explanations about what each status and command is used for, how you use it, pitfalls, etc. It would also be good to add example messages.

I think the documentation would be easier to read if it was organised into themes, e.g:

• Signal programs
• Connected equipment
• Dynamic traffic control
• Monitoring and errors

That would also quickly give readers an impression about what can be achieved with RSMP for traffic signals.

Index of all status, commands, alarms, etc can still be used as well, with links to the relevant sections.

[emiltin]

Going to split into smaller tasks