Improve signposting to OS exec in our docs

[lucyb]

We believe users aren't using OS exec when it could be useful to them.

We have a helpful how-to guide explaining OS exec here. However, we expect users are more likely to go to the page on running your code locally, where OS exec is not mentioned. Can we add a link on this page to help users find their way to the how-to guide?

The related conversation in Slack can be found here.

[iaindillingham]

See whether we can introduce opensafely exec where we introduce opensafely run.