Add list of open data sources to HC

[ilmartyrk]

We should add a list of open data sources to help-center so that users would have some verified sources to their topics/arguments etc.

[oksks]

@ilmartyrk, is that a task for Becca?

[ilmartyrk]

@oksks I think it's more general task, maybe we should start from listing any know sources under this issue and if it looks complete enough we can add it to HC.

[ilmartyrk]

List of sources:

• https://avaandmed.eesti.ee/