feat(helm): add command to generate documentation

This adds a hidden 'helm doc' command that generates documentation out
of the Helm help text.

Currently it can create:

- man pages for HELM(1)
- Markdown documentation
- bash autocompletions

Those are the three built-ins for Cobra.

The command is hidden so that it doesn't show up to the regular user,
since this is really a specialized task. It has the advantage of making
it trivially easy to install man pages and bash completion at installation time.