adding normative statement about source information

[kathryn-ods]

Overview

• What does this pull request do? adds a normative requirement that statements should include source info
• How can a reviewer test or examine your changes?
• Who is best placed to review it?

Closes #537

Translations

• New or edited strings appearing as a result of this PR will be picked up for translation
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  translating. See: https://openownership.github.io/bods-dev-handbook/translations.html

Documentation & Release

• I've checked that the documentation builds without failing. See: https://github.com/openownership/data-standard#building-the-documentation-locally
• I've thought about how and when this needs to be released. See:
  https://openownership.github.io/bods-dev-handbook/standard_releases.html
• I've updated the changelog, if necessary.
• I've updated reference.rst to reflect any changes to schema structure or naming.
• I've added or edited any other related documentation. See: https://github.com/openownership/bods-dev-handbook/blob/master/style_guide.md

[kathryn-ods]

@kd-ods imo this is the only thing in the sources and annotations page that isn't otherwise covered in the schema/docs. See comments in #537 for more detail.

[kd-ods]

If we're deleting the provenance page, I feel we need to do something with the verification-related content... and also the point about using annotations to record data processing/cleaning/enrichment.

On the first, maybe just an edit to the source.type field description, to:

"The types of the source, using the sourceType codelist. Include 'verified' in the array if the information in the Statement has undergone a verification process."

On the second, I actually think this is a bigger issue. If annotations are to be used to note data cleaning etc, then those reasons should be in the motivations codelist. BUT let's not get bogged down in that for this release. We can spin up a new issue with the title "Consider adding 'cleaning' and 'enrichment' to the motivations codelist" with some background and rationale. Then come back to it in the future.

What do you think?

[kathryn-ods]

@kd-ods have made those changes and made #638 for the annotations issue

[kd-ods]

Great. This is good to go.