Make a series of small improvements

[StevenMaude]

This is a series of small editorial and formatting improvements.

There are also some small URL fixes. Most of these are simply updating HTTP to HTTPS, but there was one broken link.[^1]

[^1]: These were detected by running lychee against the repository. I've done similar before for OpenSAFELY's documentation; it's useful to add tooling like this to catch broken links, when you're including many URLs in text. NHS Digital's Software Engineering Quality Framework repository uses a similar type of tool.

[StevenMaude]

Issues I also spotted, but won't address as not entirely sure of the correct resolution:

• The "Data Saves Lives" links possibly could be updated to the published version.
• There's a mixture of types of apostrophe in the documents: ' versus ’, for example. I don't know what your preferred house style is. Ideally, there should be just one kind in use.