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.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.50%. Comparing base (2c4bf3c) to head (ff3a85f).

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #7122      +/-   ##
============================================
- Coverage     78.51%   78.50%   -0.01%     
  Complexity     6752     6752              
============================================
  Files           759      759              
  Lines         22402    22402              
  Branches       4400     4400              
============================================
- Hits          17589    17587       -2     
- Misses         4108     4109       +1     
- Partials        705      706       +1     

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