docs: add templates & clarify local development guide

[Sathvikaundala]

This is a documentation-only PR to improve contributor onboarding for TheAlgorithms/Java:

• Adds .github/ISSUE_TEMPLATE/good-first-issue.md to help new contributors find tasks.
• Adds .github/PULL_REQUEST_TEMPLATE.md to standardize PRs.
• Appends a Local Development & contributing block to README.md.

Why:

• Low-risk, high-value changes to make it easier to contribute and speed up reviews.

How I tested:

• Docs-only; verified files updated locally.

Notes:

• Happy to adjust wording per maintainers' preferences.