Create "What is Carvel?" page

Based on a lot of feedback in the community, there should be a page that talks about what Carvel is, why it was created, and why users should use it.

For this issue, we need to
* Create the content for the page. See the hackmd for a draft of the content: https://hackmd.io/@B-b-yJ1nR0ysBlj8hrLlEw/rypbmEPJp
* Figure out where this page should live on the site. We should put some or all of this content on the home page, carvel.dev. Because of how the site is configured, the home page is currently the only place that talks about all the projects together.
* Make sure there are links to this page in the project docs to allow readers to be able to find this content as they discover Carvel

Start with the linked HackMD draft, then inspect the site home page and project documentation to decide where the shared Carvel explanation belongs. Create the page content, place it appropriately on carvel.dev, and add discoverable links from the project docs; done means the content explains what Carvel is, why it exists, and why users should use it.

Written by the indexing model from the issue text.