docs-bug(Accessibility): Clarifying Accessibility target in documentation

### Documentation Feedback

I'm evaluating component libraries to use in an enterprise app, and accessibility is a key consideration. Having looked through the documentation, I'm not able to find a specific target for Angular Material's accessibility. Would you all consider specifying which WCAG level you are targeting? PrimeNG specifically calls out that they meet the WCAG 2.1 AA standard. While Angular Material seems to put a lot of work into accessibility, it would be helpful to be able to state that the component library we use targets a specific WCAG level.

### Affected documentation page

material.angular.io

打开 material.angular.io 并查看现有的无障碍指南，找出库的目标在哪里有描述，或哪里缺少描述。与维护者确认预期的 WCAG 级别，然后在相关页面记录该目标；当用户能够清楚地识别所声明的无障碍目标时，即视为完成。

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