Update to docs: primer and gov pages

[kd-ods]

Overview

What does this pull request do?

Implements non-normative changes to the documentation:

#753
And gov page updates from #506

How can a reviewer test or examine your changes?

Who is best placed to review it?

Translations

• New or edited strings appearing as a result of this PR will be picked up for translation
• I've notified the translation coordinator of any new strings that will need
  translating. See the Handbook

Documentation & Release

• I've checked that the documentation builds without failing. See the Handbook
• I've thought about how and when this needs to be released. See the Handbook
• I've updated the changelog, if necessary. See the Handbook
• I've updated reference.rst to reflect any changes to schema structure or naming.
• I've updated the BODS validation specification if necessary. See the Handbook
• I've added or edited any other related documentation. See the Handbook

[kd-ods]

Note to self on fixing linkcheck problems:

The two broken links on standard/modelling/identifiers.rst should be fixed as follows:

1. The link text 'ICAO Document 9303 §5' should be changed to 'ICAO Doc 9303, part 3', and its url from

http://www.icao.int/publications/Documents/9303_p3_cons_en.pdf

to

https://www.icao.int/sites/default/files/publications/DocSeries/9303_p3_cons_en.pdf

2. The url for the link 'Appendix B to Part 4 of ICAO Doc 9303' should be changed from:

http://www.icao.int/publications/Documents/9303_p4_cons_en.pdf

to

https://www.icao.int/sites/default/files/publications/DocSeries/9303_p4_cons_en.pdf

3. The broken 'https://www.iso.org/iso-3166-country-codes.html' link isn't broken. The auto check is being denied access but the link will work for readers.