docs: add feature reference docs

[frydj]

Summary

Adds new reference documentation for Fluree features:

New files

• authentication.md: Auth methods, JWT tokens, DID support
• full-text-search.mdx: Lucene integration and search syntax
• reasoning.mdx: OWL inference engine reference
• shacl-validation.mdx: SHACL shapes reference
• sparql-syntax.md: SPARQL query support

Significantly expanded

• error-codes.mdx: Comprehensive error code reference
• policy-syntax.mdx: Complete policy rule documentation
• transaction-syntax.mdx: Full transaction format reference

Minor updates

• cloud-http-api.md: Small fixes

Test plan

• Verify new reference docs match implementation
• Check expanded docs cover all edge cases

[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 👈 (View in Graphite)
• 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
• 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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