Update "Shared Docs" to "Contributing"

[a-mccarthy]

The Shared Docs all seem to be related to contributing to the Carvel project. It might make it easier for folks to find this information in the navigation if it was labeled "Contributing" instead of "Shared Docs".

• Update the top navigation drop down from "Shared Docs" to "Contributing"
• Update https://github.com/carvel-dev/carvel/blob/develop/site/content/shared/docs/latest/_index.md to say Contributing
• Update the brief message on https://github.com/carvel-dev/carvel/blob/develop/site/content/shared/docs/latest/_index.md to be a little more useful.

Current content:

This documentation pertains to Carvel at a project-level such that these documents are shared across the individual tools.

Suggestion content:

The Carvel maintainers use the same code, issue triage, PR review, and release processes across all of the Carvel projects. The pages in this section have information on contributing to Carvel and each project in the Carvel toolkit. We look forward to hearing from you. If you dont have questions about the contribution processes please reach out to the community and maintainers.

[bhavyastar]

Hey @a-mccarthy , I would like to work on this issue.

[github-actions[bot]]

This issue is being marked as stale due to a long period of inactivity and will be closed in 5 days if there is no response.