Add missing newline to code generation

[lkysow]

We need to have a newline between the codegen comment and the start of
the generated docs because the markdown formatter will add it later.

Previously we'd generate the docs like:

<!-- codegen: start -->
- `global` ((#v-global)) - Holds values that affect multiple components of the chart.

But when the automatic markdown formatter ran on commit, it would add a newline:

<!-- codegen: start -->

- `global` ((#v-global)) - Holds values that affect multiple components of the chart.

This change ensure we generate the docs with that newline so when the markdown formatter runs it makes no changes. Otherwise when you ran the gen-docs make target it would always make a modification to helm.mdx and remove that newline.

[thisisnotashwin]

💥

[kschoche]

Brilliant!