Compatibility categories are not documented

[dthaler]

There was discussion in some github issue or document, where is it?
It should be documented and linked from various places like
https://gedcom.io/compatibility/ and https://magikeygedcomconverter.azurewebsites.net/Compatibility

FamilySearch/GEDCOM.io#48 is relevant but does not discuss feature categories. We currently have:

• Tree Level 1 (maximal70-tree1.ged):
• Tree Level 2 (maximal70-tree2.ged):
• Memories Level 1 (maximal70-memories1.ged):
• Memories Level 2 (maximal70-memories2.ged):
• Latter-day Saint Services (maximal70-lds.ged):

FamilySearch/GEDCOM.io#41 added files to https://github.com/FamilySearch/GEDCOM.io/tree/main/testfiles/gedcom70 but did not update https://gedcom.io/tools/#example-familysearch-gedcom-70-files

Filed FamilySearch/GEDCOM.io#308 for documented the test files.
Once that is done, then this repository can be updated to reference it.