docs: restructure and clarify the documentation set

[srikanth-bitdynamics]

Reworks the documentation so a reader new to the codebase can build understanding top-down, rather than assuming familiarity with the design.

Across concepts, guides, and reference:

• clearer hierarchy — a short lead per page, then sub-headings a reader can follow in order
• diagrams where they explain a mechanism (the settlement boundary, the constant-product swap, the connect/trade handoff, the registry boundary, the read-vs-write API split)
• short inlined Daml snippets copied from the source, next to the prose that describes them
• cross-links from each concept to the test that proves it
• plainer, more precise prose; deep or edge detail moved into per-page reference sections

Also:

• Token Standard V2 (CIP-0112) wording updated to reflect that it is merged into Splice main and is the network default
• plain-language comments added to the test files the docs link to (comments only; no test code changed)
• Daml snippets are syntax-highlighted via a Shiki language alias in the docs-site config

Verified: every doc-to-source and doc-to-test link and anchor resolves, the docs site builds, and the inlined snippets match the source.

getting-started.md is intentionally not included here (its test counts are being updated separately).