Add restore key to _freeze cache key

[penelopeysm]

This PR makes a tiny change to both preview and publish workflows:

docs/.github/workflows/publish.yml

Lines 40 to 49 in d7dd920

	- name: Restore cached _freeze folder
	id: cache-restore
	uses: actions/cache/restore@v4
	with:
	path: |
	./_freeze/
	key: |
	${{ runner.os }}-${{ hashFiles('**/Manifest.toml') }}-${{ hashFiles('**/index.qmd') }}
	restore-keys: |
	${{ runner.os }}-${{ hashFiles('**/Manifest.toml') }}

The main cache key itself will now be determined by both the Manifest as well as the contents of the notebooks (**/index.qmd). However, if the contents of the notebooks have changed, it will use the restore key to load the latest version of _freeze that was run with the same Manifest.

The aim is to let each CI run access the most recently cached _freeze folder, to avoid re-rendering the same docs page on successive CI runs.

Closes #536

[github-actions]

Preview the changes: https://turinglang.org/docs/pr-previews/537
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[mhauru]

The code makes sense to me, but I don't understand the GA caching business well. Is there a way for us to see that it's working as intended?

[penelopeysm]

Sadly, not until it's merged 🙃