Initial PR of documentaton on datatypes_conf.xml

[pvanheus]

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[pvanheus]

The aim of this PR is twofold:

1. Recommend to use only datatypes_conf.xml.sample when adding data types, thereby reducing the impact of new datatypes (i.e. no code changes needed for most new datatypes).
2. Add documentation for the datatypes_conf.xml format. This is almost certainly already wrong because of changes @mvdbeek is planning but getting a start is important. Docs were drafted using Claude.