Add links to terms from literature or examples

[anenadic]

E.g.

• primacy effect in episode 23
• Labelling diagrams, flow charts, fill-in-the-blank exercise in episode 5 (could e.g. link to Greg's Teaching Tech together book)
• concept map in episode 2

There may be a few more examples - just for completeness and further reading we should probably look to include such links.

[anenadic]

I am happy to do a PR once we agree if and which links to include.

[brownsarahm]

This is related to the broader discussion in #1223, more input is welcome

[ndporter]

Agreed this should probably wait on #1223, given both citations and external links are tied up in those decisions.