Description of your changes

Fixes #599

This adds a new template function to content called include_optional. The regular include function expects the file to exist and will panic if it doesn't. With this new function, you can provide a fallback value (or an empty string) that will be returned if the file is not found.

Example:

{{ include_optional "path/to/file" "File does not exist" }}

I have:

• Read and followed terraform-docs' contribution process.
• All tests pass when I run make test.

How has this code been tested

Tested by running make test.