Add adr template and for adr for empty subdivisions

[derTobsch]

closes #430

[derTobsch]

@XSpielinbox if you want you could take a look at this adr :)

[sonarcloud]

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[XSpielinbox]

@XSpielinbox if you want you could take a look at this adr :)

Thank you! I definitely will take a closer look as soon as I have time.

[derTobsch]

@XSpielinbox if you want you could take a look at this adr :)

Thank you! I definitely will take a closer look as soon as I have time.

Hey @XSpielinbox, i would like to merge this to gain progress.
If there is anything missing or wrong in the documentation, please let me know and we fix it afterwards :-)

[XSpielinbox]

So, finally:
Sorry, for the long delay:

1. The Context of ADR 001 is a bit brief in my opinion, but should be fine.
2. There are multiple locations where regions can be defined (the .xml files, the HolidayCalendar Enum, the .properties files). Perhaps it would be wise to clarify when a country/region code should be added where (and even if it shall be added everywhere at once, this would make sense to me to ensure that one does not forget / overlook one of the locations). One could also take this as an opportunity to clarify what consequence/use each of those entries has (e.g. the description in the .xml vs. the country_descriptions.properties).
3. The city/regional holiday handling as discussed in Improve india and moldova for ISO3166-2 #360, Assumption day is a holiday in most of bavaria #269 and Breaking: make country codes ISO 3166 conformant #267 should definitely be an ADR in my opinion.
4. I would appreciate an ADR on how to handle unreliable data like the date of New Year's Day and 2nd January or Christmas Day and Boxing Day in Scotland, as mentioned in Fix United Kingdom of Great Britain and Northern Ireland #353 or the date of Christmas Day and Boxing Day in Bermuda as discussed in Fix British Overseas Territories #342, unclear start years or the holidays in the Bahamas (see Add Bahamas public holidays #277)
5. Adding of sources as discussed in Add sources to holiday.xsd #332 should be written down in an ADR.
6. Usage of xinclude as you proposed in https://github.com/focus-shift/jollyday/pull/335/files#r1342750983 should be discussed in an ADR.
7. The adoption of ISO 3166 should be outlined in an ADR, see Breaking: make country codes ISO 3166 conformant #267 and Breaking: Move British Crown dependencies to own countries #326
8. Some holidays do not follow a regular schedule, so they have to be hardcoded, of course. But some others do, but the respective regularity is (currently) just not supported by jollyday. How to then handle these holidays should in my opinion be clarified in an ADR as well. See Add Mauritius public holidays #279, Add Singapore public holidays #283 and Add Hong Kong public holidays #288