docs: add VitePress documentation site at ldelements.org

[ddeboer]

Adds a VitePress documentation site under docs/, deployed to https://ldelements.org, and makes it the single source of truth for all documentation.

Site

• Guide: introduction pages (what/when/core concepts/architecture, with hand-drawn Mermaid diagrams), two tutorials (Build a pipeline, Build a search API with a Docker Compose setup), and 13 how-to recipes grouped by concern.
• Reference: one page per package (all 26), packages and standards overviews, and the existing decision records wired into the sidebar (titles generated from the files at build time).
• Local search, sitemap, clean URLs, edit links, dark/light themes.

Content migration (ADR 17)

• All package README content moved into docs/reference/<package>.md; READMEs slimmed to description, installation, at most one example, and a link to the package’s docs page (3,223 → 630 lines total). npm pages stay accurate because they contain little that can go stale.
• The README’s comparison table and dependency diagram moved into the docs; AGENTS.md and CONTRIBUTING.md record the new policy.

Completeness audit

All packages and four consumer projects (Dataset Register, Dataset Knowledge Graph, LODA pipeline, Linked Open Limburg) were audited against the docs; every finding was verified against source before fixing. Highlights:

• Corrected factually wrong docs: a documented option that does not exist (pipeline-shacl-sampler timeout), an outdated ItemSelector signature, stale ‘Executor’ terminology, a materialized view that is now a table, a false log-streaming claim, and schema examples that failed the indexer at boot (missing path).
• Documented previously missing surface: Stage.project, reader template substitution, SparqlUpdateWriter semantics, writer fan-out and standalone use, failure isolation, the full ProgressReporter event table, createQlever() options incl. native mode and the absolute-dataDir requirement (createQlever({mode:'docker'}) silently fails without dataDir (no bind-mount, wrong workdir) #638), QLever’s supported dump formats, registry criteria operators, verdict contracts, maxFacetValues, range facets, the GraphQL pagination contract, and a working TypeScript schema-module bundling recipe.
• Content from fix(pipeline): fail fast when the provenance store cannot persist records #662 (provenance check(), volume writability) is ported into the docs, since the READMEs that carried it are slimmed here.

Deployment

• .github/workflows/docs.yml builds the site on every PR (the build fails on dead links) and deploys to GitHub Pages on pushes to main.
• The Pages site is created with the ldelements.org custom domain attached; DNS already points at GitHub Pages, so the first deployment after merge goes live. HTTPS enforcement follows once the certificate is issued.