TINYDOC-3607: Link the Trust Center from the security guide and clarify the Accessibility Checker scope.

[kemister85]

Ticket: TINYDOC-3607

Site: Staging branch

Changes:

A reader evaluating {productname} had no route from the documentation to any compliance evidence, and the only conformance language in the documentation described the wrong subject.

modules/ROOT/pages/security.adoc

• Added a Security assurance and compliance section linking the Trust Center and describing what it holds: audit reports, penetration test reports, and a security posture questionnaire, available on request.
• Added the matching entry to the page's Overview list.

modules/ROOT/pages/a11ychecker.adoc

• Corrected the :description: metadata, which read "Checks the contents of the editor for WCAG & Section 508 accessibility problems." This is the text rendered in search results and on index cards, and an evaluator skimming it can read it as a conformance claim for the editor. It is not one — the plugin evaluates content authored in the editor.
• Added a note stating that the plugin evaluates authored content, does not evaluate the accessibility of the {productname} user interface, and that the rules it applies are not a statement of conformance for {productname} itself, with a cross-reference to the Accessible navigation guide.

Pre-checks:

• Branch is correctly prefixed:
• Files have been included where required (if applicable).
• Build passes without console errors, warnings, or issues.

Review:

• Documentation Team Lead has reviewed.

[metricjs]

LGTM for a quick win. Nitpick: might be worth calling it out more obviously? E.g. at top of page. Can be something for another day though if we like.