Add option in `make-live.R` to only render

In reference to the RNA-seq/00c-tximeta-other-species.Rmd, it would be nice if it was rendered automatically but it is not a document that will be covered in lectures so we don't need a `live` version:

> If we add it to the rendering list, we will need an option to not do the `-live` part. We would also need to add its files to S3, which is not necessarily a problem... It is probably worth filing an issue to add an option to `make-live.R`, but for now manual should be fine.

_Originally posted by @jashapiro in https://github.com/AlexsLemonade/training-modules/issues/379#issuecomment-786029680_

Start by reading make-live.R and the referenced RNA-seq/00c-tximeta-other-species.Rmd to understand how documents are selected and rendered. Done means the script has an option to render the document without creating its -live version, while preserving the existing rendering behavior.

Written by the indexing model from the issue text.