feat: add new tutorials

[AzulGarza]

this pr adds two new tutorials: how to build custom ensembles and how to enable explainability of them.

[Copilot]

Pull request overview

This PR updates the MkDocs site navigation to surface two tutorials focused on (1) building custom ensembles and (2) explaining/comparing foundation models and ensembles via the TimeCopilot agent.

Changes:

• Adds navigation entries for custom-ensembles and explaining-foundation-models-and-ensembles example notebooks.
• Introduces a new notebook tutorial demonstrating model/ensemble explainability via natural-language queries.
• Updates the site copyright year to 2026.

Reviewed changes

Copilot reviewed 2 out of 3 changed files in this pull request and generated 3 comments.

File	Description
mkdocs.yml	Updates docs navigation (adds the new tutorials; also removes several existing examples from nav) and bumps copyright year.
docs/examples/explaining-foundation-models-and-ensembles.ipynb	New tutorial notebook showing how to register multiple forecasters (including an ensemble) and query the agent about results.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.