docs: onboarding — add issue & PR templates, clarify README local dev and .env.example

[Sathvikaundala]

This is a small documentation-only PR to improve contributor onboarding:

• Add .github/ISSUE_TEMPLATE/good-first-issue.md to help new contributors find tasks.
• Add .github/PULL_REQUEST_TEMPLATE.md to standardize PR info and ease reviews.
• Clarify README Local Development troubleshooting (tips for host/port, node_modules, Docker).
• Add explanatory comments to .env.example.

Why:

• These are low-risk, high-value changes that reduce friction for new contributors and make reviews faster.

How I tested:

• Document-only changes; no code changes. Verified files locally.

Notes:

• If any maintainers prefer the README change consolidated into an existing section rather than appended, I'm happy to update the PR per feedback.

[CLAassistant]

All committers have signed the CLA.