Add a Helm overview page to the docs

[paigecalvert]

This issues tracks work to add a new page to the introductory section of the docs that describes the Helm project, its goals, core concepts, user personas, and key use cases, clearly addressing the who/what/when/why for Helm.

This aligns with the criteria identified by the CNCF techdocs team for graduated projects for "high level conceptual content": https://github.com/cncf/techdocs/blob/main/docs/analysis/criteria.md#assessment-criteria-and-examples

Adding an overview page was originally proposed in https://github.com/helm/community/blob/main/hips/hip-0024.md