Add section label to contributing page index.rst

[RDaxini]

• Closes #xxxx
• I am familiar with the contributing guidelines
• Tests added
• Updates entries in docs/sphinx/source/reference for API changes.
• Adds description and name entries in the appropriate "what's new" file in docs/sphinx/source/whatsnew for all changes. Includes link to the GitHub Issue with :issue:`num` or this Pull Request with :pull:`num`. Includes contributor name and/or GitHub username (link with :ghuser:`user`).
• New code is fully documented. Includes numpydoc compliant docstrings, examples, and comments where necessary.
• Pull request is nearly complete and ready for detailed review.
• Maintainer: Appropriate GitHub Labels (including remote-data) and Milestone are assigned to the Pull Request and linked Issue.

Quick fix: Link to the contributing section on the package overview section led to the contributing on the pandas webpage rather than our contributing section. Added a section label to the index of our contributing page to fix this.

[kandersolar]

Good catch, looks like we accidentally lost that label in #2219.

I wonder if there is some way to disable intersphinx for everything except :py: references, so that cases like this would result in build log warnings instead of falling back to linking to another package's docs that happens to have a matching label. Perhaps something to look into if someone has the motivation.