Proposal: Go templates alternative in Nelm

[shurup]

An alternative to Go templates has been one of the most requested and discussed features for Helm and our Nelm. Seeing the community's interest in it and even their readiness to contribute to its implementation, we created a formal proposal for the feature.

This document explains why such an alternative is needed in the Kubernetes ecosystem, provides an analysis of existing languages, and outlines the design for its future implementation in Nelm.

Please feel free to join an ongoing discussion in this PR on GitHub!

P.S. There is a Reddit thread dedicated to this matter as well.