docs(contributing): reporting a UI problem and accessibility checks

[Bugs5382]

What and why

Adds two contributor-facing pages for cryptos-web's development tooling, under a new "Contributing" sidebar category:

• Reporting a UI problem: how to turn on the development-only "Copy for UI issue" button (the console build flag and the manager's env flag, shown in Linux/macOS and PowerShell), what the copied bundle holds and never holds, and a callout that the button never reaches a release build.
• Accessibility checks: what axe covers on every kit component, why color-contrast and target-size are off in the jsdom test environment (and checked manually instead), the eslint-plugin-jsx-a11y lint gate, and the manual WCAG 2.2 AA pass each screen wave goes through.

Both facts were confirmed against the merged code on cryptos-web's main branch and its AGENTS.md.

Refs CryptOS-PKI/cryptos-web#163

Test plan

• npm run build (Docusaurus production build) succeeds and both new pages render under /docs/contributing/
• npm run typecheck passes
• task license reports no missing headers