Update Identifier Accessibility Statement link text guidance

[thisisdano]

Summary

Updated Accessibility statement link text guidance. Updated the Identifier guidance to use the text "Accessibility statement" for the required link to an accessibility statement. We improved the supporting links that describe accessibility statements.

Related issue

Closes uswds/uswds#5092

Preview link

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Problem statement

We'd like to support teams adding Accessibility statement links to their sites. Our existing guidance is a little bit ambiguous (pointing to "parent agency"), and we can clear it up by saying that these links should point to "your website or service's accessibility statement, or to your agency's accessibility statement if your website or service does not have its own." We should link to Section 508's accessibility statement guidance, and point to a good example of an accessibility statement, like the one on DHS.gov.

Testing and review

1. Read the Accessibility statement guidance on the Identifier's component page.
2. Check for grammar and sense.
3. Assure that the accessibility statement link goes to Section 508's accessibility statement guidance (https://www.section508.gov/manage/laws-and-policies/website-accessibility-statement/).
4. Assure that the example link goes to the DHS accessibility page (https://www.dhs.gov/accessibility).
5. Check the changelog entries to assure that they are clear and correct, and link to the proper PRs.

[thisisdano]

This will fail in Circle because of broken links to the USWDS 3.5.0 release in the changelog.