docs: restructure the site into five tabs and a grouped guide

[raederan]

Overview

• Header reduced to five tabs: Introduction | Guide | Use Cases | Rule Catalogue | Specification. Home is now reached through the logo top-left.
• Home links to Introduction, Guide, Use Cases and Specification.
• Get Started merged into the Introduction as its final section (/introduction/#get-started).
• Guide sidebar regrouped into OO-LD Concepts (Basics, Composition, Schema Instances, Identification & Versioning, Extensions, Meta-schema), Tooling, Mappings, Migration and Related Work.
• Mappings split into one page per format (AAS, SAMM, LinkML, NOMAD, Dlite, REST-API-LD, WoT, SPDX, Croissant, OCA, TreeLDR, yml2vocab). The /mappings/ URL is preserved.
• reference.md renamed to related-work.md, subsections flattened, Registry and Discussion moved to the end.
• Inbound links, the guide overview and the README updated to match.

Notes for review

• Restructuring only: content was moved verbatim, verified by reconstructing every split page back into its original section byte-for-byte. Only a few "this page" to "this section" fixes, plus a new list on the guide overview covering its wider scope.
• /get-started/ and /reference/ URLs go away (no redirect plugin). Pre-1.0 and mike-versioned, so released versions keep their links.
• make check passes; docs/rules.md unchanged. Nothing here touches Recommendations for template languages to generate OO-LD schemas #139.

@simontaurus
Closes #140

@LukasOro
Sync with #139