Add SonarQube accessibility rules

I checked, rule by rule, which of SonarCloud's 116 accessibility-tagged rules
(sonarcloud.io/organizations/xwiki/rules?tags=accessibility, covering HTML, CSS,
JavaScript and TypeScript) are active in the "Sonar way" profiles we actually use for
xwiki-platform. All 116 are active, except 6 HTML rules that are DEPRECATED (Flash wmode,
ismap, non-consecutive headings, link-to-image, fieldset-without-legend, table-without-caption)
and correctly excluded.

One caveat: on xwiki-platform itself, HTML and TypeScript aren't analyzed at all today (0 lines of
web/ts in the project's language distribution, only java, js, css and xml show up), so
those rules never get a chance to run there in practice. JS and CSS are analyzed and do report
other issues, just none tagged accessibility so far.

So this is already fully covered by our current setup, on top of the Docker functional tests we
already run with axe-core against real pages. No SonarCloud configuration change is needed.

Closing as already covered.