Add docs contributing information to the Contributing guide

[a-mccarthy]

In addition to how to contribute to code, this guide should cover how to contribute to docs.

Consider updating the contributing guide, https://carvel.dev/shared/docs/latest/contributing/, or the Development guide to include information on contributing to docs. Or make a new page in this section for contributing to docs.

Some things to consider including:

• Where the docs and site content is located in the repos
• If someone sees an issue in the docs, where should they make the issue? In the main carvel repo, or in the project repo?
• Which branch to make changes against. Are contributors expected to update just the development (main) branch, or do they need to update all the versions which are impacted?
• What tools and versions are used for the site
• Any special instructions for building and local version of the site for development
• Any tips or tricks to the website that someone might need to know about when contributing

[a-mccarthy]

closing this in favor of the original issue: https://github.com/carvel-dev/carvel/issues/661. I moved over my comments to the other issue as well