Publish an accessibility statement page

[brylie]

The site has no accessibility statement, so readers who hit a barrier have no way to report it. Publish a page that says what standard we aim for, what's known to fall short, and how to reach a person. The editor needs to supply or approve the wording.

Grounding: docs/accessibility-audit-2026-09.md, "Remaining recommendations (not fixed in code)", item 8

Acceptance criteria:

• A page editable in Wagtail, with a conformance target (WCAG 2.2 AA), known limitations, and a contact address or form
• The editor approves the text
• The known limitations from the audit are listed until they are fixed
• Linked from the footer on every page