Update the contributing guide to include more information about contributing

[a-mccarthy]

This is a sub-issue for #744.

We should review and update the contributing guide to include any missing information.

This includes

• adding links to docs that provide more information/resources for contributing like the proposal template, https://github.com/carvel-dev/carvel/tree/develop/proposals.
• add how to contribute to docs
• Make it clearer on the contributing guide on the website that there are many different repos and the contributing processes apply to all of them. Outline what types of things go in the Carvel repo, and what goes into specific tool repositories
• Is there an active issue backlog? Is there a roadmap?
• Add a more clear link to the Development Guidelines page. https://carvel.dev/shared/docs/latest/development_guidelines/. Is there any content in the development guidelines that someone will need if they are contributing to areas of the project that are specifically "developer" related.

[github-actions]

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.