Add app-building tutorial section to docs

[namedgraph]

Summary

Adds a ten-stage Tutorial section to the docs that rebuilds the Northwind Traders demo from an empty dataspace to a published app:

1. Hello dataspace — title, description and first content block on the root document
2. Structure — the container hierarchy
3. Model — namespace ontology import via the admin app
4. Data — CSV imports, with a walkthrough of the mapping query's document-minting BIND pattern
5. Media — content-addressed file uploads and foaf:depiction linking
6. Insight — queries, charts and views as resources
7. Composition — assembling the landing page from rdf:_N blocks
8. Publish — the public-read ACL authorization
9. App as a repository — the folder→document convention, idempotent replay, make install
10. Beyond low-code — exit ramp to the XSLT/Docker/Java extension layers

Design notes

• Each stage ends with a visible result and a "Next:" link; the container carries a hand-curated ordered TOC instead of ldh:ChildrenView, since stage order is authored.
• Steps are dual-track — "In the browser" links out to the user-guide pages, "From the command line" shows commands lifted verbatim from the demo's install scripts (nothing invented).
• Screenshot slots are marked with <!-- screenshot: ... --> comments for later automated capture.
• All documents pass trig --validate and their XHTML literals pass xmllint; the section renders cleanly through ttl-to-html.sh (11 pages, picked up by nav and sitemap).

Known follow-ups (not in this PR)

• Nav orders sections alphabetically, so Tutorial lands after Reference; moving it next to Get started needs a sort tweak in ttl-to-html.xsl.
• add-file.sh now appears both in the Media stage and user-guide/upload-file — a known drift surface to watch.

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