`pages-build-deployment` is still using Node.20 actions

This is a placeholder issue which we cannot fix, but I'll leave it open for documentation issues

The `pages-build-deployment` action, which is used to build GitHub Pages, is still using Node.js 20 actions. This causes warnings on every deployment of circuitpython.org, like:
```
build
Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@v4, actions/upload-artifact@v4. ...
```
This action is not in https://github.com/actions, so there's no way to file an issue on it.

I spent a while trying to track this down due to something in our own github actions, but ultimately figured out it was not our fault.

https://github.com/orgs/community/discussions/193381 rightly complains about this. I have subscribed to that discussion.

Start with the `pages-build-deployment` action and the linked GitHub Community discussion; the issue says the action is outside the `actions` organization and cannot be changed here. Determine whether this repository needs a documentation update, and consider the work done only when the Node.js 20 warning and its external ownership are accurately documented.

Written by the indexing model from the issue text.