Add toy 1-D regression illustration

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No files, tests, or details are provided in the issue. Start by scanning the repo for where toy demos are documented (README, docs/, examples/, or similar). Add a 1-D regression illustration in the existing examples/tutorial structure used there and connect it where other illustrations are linked. Mark done when the new illustration appears in the docs/example index and local checks (tests or docs build, if defined) pass.

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