Update OpenSAFELY tutorial documentation

[KatieB5]

I recently completed the OpenSAFELY getting started tutorial here.

There are some parts which require updating, and some areas where we could potentially improve the wording to make things clearer for our new users. See this document for a list of updates and suggestions. Please note: I have made suggestions to update wording but I am definitely not tied to these and welcome alternatives (comment functionality in enabled in the document).

The suggestions in the document above are broken into subsections based on the corresponding section of the tutorial, and there are screenshots for further context.