docs: add accessibility testing guide for plugin-e2e

[fastfrwrd]

What this PR does / why we need it:

Adds a dedicated "Accessibility testing" guide to the E2E testing docs, covering the scanForA11yViolations fixture and the toHaveNoA11yViolations matcher. Until now these APIs were only mentioned as a bullet in the @grafana/plugin-e2e README, so there was nowhere to point plugin authors.

The guide covers everything listed in the issue:

• Installing the optional @axe-core/playwright peer dependency.
• Scanning a full page, including the reminder to wait for content and where to find the axe JSON attachments in the Playwright report.
• Scanning a subtree with include / exclude, plus a note that they take CSS selectors rather than Playwright locators or Grafana end-to-end selectors, and a helper for converting the latter.
• The default Grafana rule set (DEFAULT_A11Y_TAGS).
• Running custom rules through options, with a warning that options replaces the default WCAG tags, since AxeBuilder.options() overwrites what withTags() set.
• Excluding a rule, both at scan time (options.rules) and at assertion time (ignoredRules), and why you'd pick one over the other.
• Using threshold to hold the line on a plugin with existing violations.

Which issue(s) this PR fixes:

Fixes #2484

Special notes for your reviewer:

• Every code sample was type checked against the real plugin-e2e types, and the Docusaurus production build passes with no broken links.
• The toCssSelector helper in the "Scan part of a page" section exists because resolveGrafanaSelector isn't part of the public API. If we'd rather export it, I'm happy to do that in a follow-up and simplify this section.