Expand coverage of documentation

Main tasks

[x] Revisit documentation best practices resource for inspiration w.r.t. organisation
[x] Split documentation based on above findings
[x] Update installation instructions to indicate availability on PYPI
[x] Write tutorial
[ ] Provide PTL / properties reference
[x] Provide API reference
[ ] Provide extension API reference
[x] Retain planned feature list in readme.md?
[ ] Unify the terminology used based on the nomenclature established in the PDW abstract

[x] Mention how structure checking is useful for processor testing and manual generation, but can be turned off for performance
[ ] Mention how although the reference implementation is in Python, the API and use of XPath makes it easy to port to other languages.
[ ] Mention how to use fill with node-sets in a multi context
[ ] Mention how to use fill formatting