overview document for available custom commands in lib/

[lsf37]

We have lots of custom commands and automation available in lib/ but they are hard to discover, some are undocumented, and are therefore in danger of falling out of use or being reinvented.

We should at the very least an overview document that summarises what each command does in a paragraph (still short, but more detail and less cryptic than what print_commands will give you).

This could be an .md file in docs.