Fix broken contributing guidelines link

[Copilot]

The CONTRIBUTING.rst file referenced /contributing.html which returns 404. The docs are built with MkDocs (not Sphinx), which serves pages at /contributing/ instead of /contributing.html.

Changes

• Updated URL in CONTRIBUTING.rst from contributing.html to contributing/

Original prompt

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This section details on the original issue you should resolve

<issue_title>[Bug]: Link to "Contributing guidelines" is broken</issue_title>
<issue_description>### Actual Behavior

404 - Not found is shown

Expected Behavior

GitHub points to https://github.com/python-openapi/openapi-core/blob/master/CONTRIBUTING.rst

That points to https://openapi-core.readthedocs.io/en/latest/contributing.html

This shows a 404

Steps to Reproduce

Follow the link https://github.com/python-openapi/openapi-core/blob/master/CONTRIBUTING.rst

OpenAPI Core Version

N/A

OpenAPI Core Integration

N/A

Affected Area(s)

docs

References

No response

Anything else we need to know?

No response

Would you like to implement a fix?

None</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes [Bug]: Link to "Contributing guidelines" is broken #1087

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