Add sorting by slug

[bemyak]

In https://github.com/getzola/zola/pull/1917 we discussed that sorting by slug probably makes more sense than by filepath, so here is the implementation.

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Sanity check:

• [X] Have you checked to ensure there aren't other open Pull Requests for the same update/change?
• [X] Are you doing the PR on the next branch?
• [X] Have you created/updated the relevant documentation page(s)?