"Glossary" of recurrent items that are not central to analyses

There's things that pop up time and time again in our notebooks but aren't necessarily central to the analysis:

Some examples:
- set.seed()
- the magrittr pipe
- sessionInfo()
- `if (!dir.exists("plots")) {}`

One option is to describe each of these things every time they come up, but what might be a nice alternative is having a link to a "glossary" of sorts where we explain what those items are. That way a user who understands what that's about can focus on the analysis, but a user who's confused can easily find more info and an explanation about what that thing is.

Review the recurring notebook items listed in the issue: set.seed(), the magrittr pipe, sessionInfo(), and the plots-directory check. Define the glossary’s scope and explanations, then ensure the relevant notebook references can direct confused users to it.

Written by the indexing model from the issue text.