Document TOML, YAML and JSON functions

[sheisnicola]

Please see:

• https://github.com/helm/helm/issues/7888 (incorrectly closed as a duplicate)
• https://github.com/helm/helm-www/issues/489#issuecomment-729863024

Helm defines a few functions of its own in pkg/engine/func.go for serializing and deserializing data

https://github.com/helm/helm/blob/b735abe8371359fcf0b37de8014bfafcceb1bcf8/pkg/engine/funcs.go#L50-L57

I suggest we add documentation about these functions along with the documentation for the include, required and template functions as well as in the chart template guide section about template functions.

I suggest that the following links also be referenced in the documentation, or the omissions from the Helm documentation be fixed. The text/template documentation is especially useful as it describes some aspects of value assignment and syntax that are completely absent in the Helm docs:

• https://masterminds.github.io/sprig/
• https://godoc.org/text/template

[bacongobbler]

Hi @sheisnicola. I agree that documentation surrounding these functions built into Helm's template engine would be useful. Please feel free to provide a PR to document their behavior.

If I understand correctly, https://pkg.go.dev/text/template is the new canonical location for any standard library docs (godoc.org now redirects to that location), so I would suggest we point users there instead of godoc.org.

[yxxhero]

@sheisnicola Can you create a PR for this. Thanks very much.

[nofearOnline]

@yxxhero #1309