docs: add modeling guides

[frydj]

Summary

New Learn > Modeling Your Data section with 6 guides:

• classes-and-properties.mdx: Defining types and attributes with RDFS
• building-ontologies.mdx: Creating domain models
• working-with-ontologies.mdx: (moved from guides/) Using existing vocabularies
• shacl-validation.mdx: Data validation with SHACL shapes
• semantic-vocabularies.mdx: (moved from foundations/) Schema.org, FOAF, etc.
• introduction-to-reasoning.mdx: OWL inference basics

Removed

• guides/working-with-context.mdx: Content consolidated elsewhere

Test plan

• Verify SHACL examples validate correctly
• Check ontology examples are consistent across guides

[frydj]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• docs: add feature showcase examples #71
• docs: add domain example datasets #70
• docs: add feature reference docs #69
• docs: reorganize cookbook #68
• docs: restructure querying reference #67
• docs: restructure HTTP API reference #66
• docs: add advanced topics #65
• docs: add security guides #64
• docs: add modeling guides #63 👈 (View in Graphite)
• docs: add working with data guides #62
• docs: reorganize learn foundations #61
• docs: add getting started guides #60
• docs: add project config & infrastructure #59
• main

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