Document of runners needs conversion to Hugo

The documentation on runners shows some unrendered language specific markup here:

https://beam.apache.org/documentation/runners/direct/

One see {:.language-py}.

These look like forgotten markup from another template system. I had a quick go at attempting to modify those but it's not obvious how to do it properly.

These threw me off while reading the documentation.

Furthermore, it would be really useful to document the distinction between the DirectRunner and FnApiRunner, which is only available by browsing the source code. The distinction in the documentation isn't made clearly and it's confusing.