Add subject inclusion/exclusion filter exercise

This PR adds a new tutorial exercise covering subject inclusion and exclusion filters in Annif (introduced in v1.4).

What's included

Three practical examples using STW and YSO vocabularies:

• Exclude by RDF type — removing Thsys nodes from STW so only descriptors are suggested (exclude_type=zbwext:Thsys)
• Exclude by URI — dropping specific high-frequency head labels such as Theory and United States from STW
• Include by concept scheme — restricting a project to geographic terms only using the YSO places scheme (include_scheme=)

All examples use the tfidf backend and projects.toml configuration format.

Related

Covers the filter syntax introduced in NatLibFi/Annif#846.