Improve Documentation

[kmuehlbauer]

We now have a minimal documentation, yikes https://docs.openradarscience.org/projects/xradar/ It already looks very nice, but can be improved.

What's missing list (please extend):

• [x] mention current version string in docs #19
• [x] mention conda-forge package #19
• [x] update installation guide #19
• [x] update contributing guide #19
• [x] direct links to the current standards #24
• [x] description of the data model (mandatory, optional) #24
• [x] fleshed out description how the importers are working #20
• [x] showcase of implementation of an example accessor #21
• [x] tutorial on producing PPI plots #28
• [ ] tutorial on producing RHI plots
• [ ] how to overlay other types of data (ex. model)
• [ ] using hvplot to create interactive plots
• [ ] xmovie example
• [ ] tbc