Docs: The Helm read the docs site doesn't have the template documentation

[mattreyuk]

I noticed the generated "read the docs" site doesn't have the documentation on templates. I think this is just because it's missing in mkdocs.yml. I plan to try and fix this myself and provide a pull request.

[bacongobbler]

Good catch. There seems to be a few others missing too. The Helm Guide to Writing Awesome Charts, Helm Generate and Template, and Helm Plugins are all missing from mkdocs.yml.

[mattreyuk]

I'll make sure to add them in as well.