[WCAG2.1]: Add the VA Mobile team's accessibility test plan to VSP documentation

[1Copenut]

Issue Description

The VA Mobile team wrote an awesome accessibility testing plan based on WCAG2.1. They have been using it as their baseline for building an accessible app. We would like to add it to VSP accessibility documentation.

Tasks

• [ ] Port the linked test plan above to VSP accessibility documentation
• [ ] Credit the VA Mobile team correctly
• [ ] PR and merge the plan with at least one VA Mobile team member as a reviewer

How to configure this issue

• [ ] Attached to a Milestone (when will this be completed?)
• [x] Attached to an Epic (what body of work is this a part of?)
• [x] Labeled with Team (product support, analytics-insights, operations, service-design, tools-be, tools-fe)
• [x] Labeled with Practice Area (backend, frontend, devops, design, research, product, ia, qa, analytics, contact center, research, accessibility, content)
• [x] Labeled with Type (bug, request, discovery, documentation, etc.)

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[1Copenut]

This issue has been completed. We've added the WCAG 2.1 success criteria to the master spreadsheet of Platform standards. Moving to closed.