Example-product docs are used in links but no longer available

Starter Pack uses intersphinx links examples in [MD](https://github.com/canonical/sphinx-docs-starter-pack/blob/main/docs/reference/myst-syntax-reference.md?plain=1#L365) and [RST](https://github.com/canonical/sphinx-docs-starter-pack/blob/main/docs/reference/rst-syntax-reference.rst?plain=1#L425) syntax references.

But the example product docs project is no longer available: https://canonical-example-product-documentation.readthedocs-hosted.com/

We should update the examples to link to something existing.