Add trick for accessing chart names inside Values with dashes to Best Practices documentation

[MurzNN]

We have a pretty old issue https://github.com/helm/helm/issues/2192#issuecomment-892738573 that was closed, but no documentation is improved by it. So I decided to open a new issue to fix the documentation, because I spend a lot of time tinkering with "dash in chart name" problem before found that issue :(

Let's add to the page https://helm.sh/docs/chart_best_practices/values/ any note about restriction of dashes in variable names and the workaround using "index" function:

{{ index .Values "gitlab-runner" "checkInterval" }}

Or maybe anybody can suggest a better place to describe this limitation and workaround?

[bacongobbler]

transferred to helm-www.

Source: https://github.com/helm/helm-www/blob/main/content/en/docs/chart_best_practices/values.md

[ebuildy]

So we shouldnt be able to name chart with "-" ?