Fix broken link to Contributor's Guide in documentation

[vidipsingh]

Description

Modifies docs/index.rst to use button-ref instead of button-link, ensuring the Contributor's Guide button links to source/user_guide/contributing.html

Fixes: #4911

Type of change

Please add a line in the relevant section of CHANGELOG.md to document the change (include PR #)

Important checks:

Please confirm the following before marking the PR as ready for review:

• No style issues: nox -s pre-commit
• All tests pass: nox -s tests
• The documentation builds: nox -s doctests
• Code is commented for hard-to-understand areas
• Tests added that prove fix is effective or that feature works

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.71%. Comparing base (2dcc015) to head (d13dfa2).
Report is 1 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #4913   +/-   ##
========================================
  Coverage    98.71%   98.71%           
========================================
  Files          304      304           
  Lines        23478    23478           
========================================
  Hits         23176    23176           
  Misses         302      302           

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