Add missing advanced features documentation page

[Copilot]

Multiple documentation files reference advanced.md which doesn't exist, creating broken links throughout the docs.

Changes

• Created docs/advanced.md with comprehensive coverage of all four advanced features:
  • Streaming XML with XPath (links to existing streaming-xml.md)
  • Python Functions in Transforms (links to existing python-functions.md)
  • SPARQL Support (full documentation, no dedicated page exists)
  • SHACL Validation (full documentation, no dedicated page exists)

Each section includes key features, code examples, and usage guidance. All Turtle code blocks have complete namespace prefix declarations.

Example

@prefix setl: <http://purl.org/twc/vocab/setl/> .
@prefix prov: <http://www.w3.org/ns/prov#> .
@prefix sd: <http://www.w3.org/ns/sparql-service-description#> .

# Load transformed RDF to SPARQL endpoint
:sparql_load a setl:Load, sd:Service ;
    sd:endpoint <http://localhost:3030/dataset/update> ;
    prov:used :myGraph .

Fixes broken links in: README.md, tutorial.md, quickstart.md, python-api.md, examples.md, jsldt.md

Original prompt

The advanced features page mentioned in many docs pages seems to be missing. Add the page to list the ones mention in other pages.

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