Added contribution docs + issue templates + PR template

[shashwat4543]

Summary

Improves contributor onboarding: expands the README's contribution
workflow, adds a CONTRIBUTING.md, makes issue templates more actionable,
and adds accessibility/content checks to the PR template.

Changes

• README.md — "Contribution workflow" section expanded into
  "Contributing," with a first-time-contributor quick start, an
  8-step workflow, and a "Getting help" section.
• .github/CONTRIBUTING.md (new) — setup steps, coding/JSON-schema
  conventions, accessibility standards, and manual testing steps.
• .github/ISSUE_TEMPLATE/config.yml (new) — disables blank issues
  and points contributors to the contributing guide or an existing
  issue instead.
• .github/ISSUE_TEMPLATE/bug-report.yml — added required page/URL
  and browser/device fields, plus a severity dropdown, so reports
  don't need a reviewer round-trip for basic info.
• .github/ISSUE_TEMPLATE/feature-request.yml — added a required
  "accessibility considered?" field.
• .github/ISSUE_TEMPLATE/content-task.yml — added a required
  effort estimate so newcomers can pick tasks that fit their time.
• .github/PULL_REQUEST_TEMPLATE.md — split into type-of-change,
  Accessibility checks, Content checks, and Testing sections.

Acceptance criteria

• README includes contribution workflow
• Issue templates are actionable
• PR template includes accessibility/content checks

Notes

• Assumes a good first issue label exists (referenced in the README
  and content-task template) — add it if it doesn't yet.
• No behavioral/site changes — docs and .github/ templates only.

Closes #26