docs: restructure runtime docs by reader intent

[bartlomieju]

The runtime sidebar grouped pages under "Fundamentals" and "Reference
guides", but the split was inconsistent: the same topic was scattered
across both groups (TypeScript, testing, linting, and HTTP each appeared
twice), and "Reference guides" was mostly how-to content sitting next to
genuine reference. A reader could not predict which group to check for a
given topic.

This regroups the runtime docs by what the reader is trying to do — Get
started, Guides, Concepts, Advanced, and Reference. Guides is kept to a
short list of high-traffic tasks so it stays scannable and does not bury
Concepts; the long tail of how-tos moves to Advanced. The top-bar "Docs"
link is renamed "Runtime" (Examples and the API reference are docs too),
and the lint-rule catalog gets a Reference link so it is no longer
reachable only via search.

The regrouping itself changes no page URLs. On top of it, the dependency
management content (adding, versioning, overriding, vendoring, lockfiles,
supply chain, publishing, private registries) moves out of the Modules
concept into a new Dependency management guide, so the Modules page is a
lean concept and the guide is a task-oriented reference. It keeps the
relevant oldUrl redirects, and inbound links are updated to the new
location.