Apply simple on-boarding feedback from SE-2116

Mainly spelling, grammar and fixing links, but also including some small rewording and reorganisation.

I did this as carefully as possible, but the diffs are showing more changes than ideal:

1. The merge has dropped a reference to the section time_management_and_invoices in the newcomer chapter definition. This section was not in the production or dev courses and I can't see a corresponding file, so I'm assuming that omission a "good thing" and I'll leave that as is.

2. I used an ironwood devstack to match production, assuming that would reduce any conflicts. Some of my edits using the GUI editor have added spurious changes (hash colour codes instead of rgb()), so I think earlier changes may have been made using a different version.

3. Some of the previous content looks like it has been edited in an external editor rather than the built in one that I used. This has made some of the diffs a lot bigger and harder to understand.

4. The merge has also added a videos directory and removed the inline video definition from vertical/opencraft_task_management_resources.xml. This is possibly a good thing.