Tooling to discover (and prune) orphaned/unused images, and other asset files

[nuke-web3]

There are many files in this repo, and over time content is evolving in various ways, and who maintains sections changes too. It is getting difficult understanding how and if all are needed for a cohort, and more practically if files are needed at all (these being almost exclusively images).

We want methods to help us identify files that may be orphaned unintentionally.

Perhaps something like https://github.com/akkana/scripts/blob/master/weborphans accessible as a yarn command would be helpful for:

• finding and optionally removing (image) files that are not linked in any slide deck.
• identifying when READMEs with lesson plans don't link internally to slide decks, indicating the deck may not be relevant in the module for a specific cohort at least (related to Cleanup contracts and blockchain module post presentation #782 that I think was not using a tool to assist in cleaning up)