Update to Real World Identifiers page and related content

[kd-ods]

Overview

What does this pull request do?

• Updates the Real World Identifiers page
• Adds an example plc entity statement file
• Adds that example to the Example Data page

Note: there's a build warning because of an outdated link on the reference.rst page. We'll resolve that as part of #528

How can a reviewer test or examine your changes?

Build the docs, read the page, check the links. Run pytest

Who is best placed to review it?

@kathryn-ods

Closes #530

Translations

• [x] New or edited strings appearing as a result of this PR will be picked up for translation
• [x] I've notified the translation coordinator of any new strings that will need translating. See: https://openownership.github.io/bods-dev-handbook/translations.html

Documentation & Release

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

[kd-ods]

@kathryn-ods - this is ready for rereview. Can you build it locally and re-read. I skipped that bit myself :-/