docs: add issue and pull request templates

[nonggde]

Summary

Closes #71.

• Added concise bug report and feature request templates under .github/ISSUE_TEMPLATE/.
• Added a repository pull request template with lint, test, build, documentation, and accessibility checks.
• Updated the README contributing section with the type/area-slug branch convention, template locations, and StableRoute Discord link.

Testing

• Not run: documentation/template-only change with no runtime code paths changed.
• Verified template paths and README references locally.
• Verified the referenced scripts exist in package.json: npm run lint, npm test, and npm run build.

Rendered Preview Notes

• The issue templates include GitHub Markdown front matter for Bug report and Feature request selections.
• The pull request template is a standard Markdown checklist and should render automatically for new PRs.