fix(docs,ui): repair docs navigation and the collapsed admin sidebar

[hoalongnatsu]

What does this PR do?

Fixes three navigation bugs — two on the docs site, one in the admin UI — that
each made part of the product hard to move around in. Also adds a step-by-step
SigNoz logs guide.

Why?

All three were reported by users, and they share a shape: navigation that
looked fine but silently went nowhere.

• Docs sidebar never showed the current page. No sense of place while
  reading.
• In-page section links did nothing. The docs cross-reference themselves
  heavily; every one of those links quietly failed. A follow-up report showed
  links to numbered headings failed for a second, unrelated reason, which
  affected 13 links across 5 files.
• The collapsed admin sidebar hid most of the navigation. Each group icon
  linked only to that group's first item, so collapsing the rail made
  everything else unreachable. Users could not tell items were missing —
  the rail looked complete.

How to test

Docs site — serve versus-incident/src/ and check:

1. Navigate between pages; the left menu highlights the current page and
   expands the section containing it.
2. On a page with in-page links (agent/data-sources/prometheus), click one:
   it scrolls to the section and the URL gains ?id=….
3. Click a link to a numbered heading (enterprise/metrics/signoz →
   "What it discovers"): it scrolls rather than sitting still.
4. Paste a ?id=… URL directly into the address bar: it lands on the section,
   not the top.
5. Ordinary page-to-page navigation still starts at the top.

Admin UI

cd versus-incident/ui && npm test    # Sidebar suite, incl. the new flyout test

Then collapse the rail and hover a group icon: the flyout lists every item in
that group. Move diagonally into the panel — it stays open. Tab to a group
icon: the flyout opens; Escape closes it. Check a bottom group (Manage)
where the panel is clamped against the viewport edge.

Type of change

• Bug fix (non-breaking)
• New feature (non-breaking)
• Breaking change (config / API / default behavior)
• Documentation only
• Refactor (no functional change)
• CI / build / chore

Mostly a bug fix. It also adds one new documentation page, so it is not
strictly "documentation only" either.

Checklist

• go test ./... passes locally — no Go code changed; not run
• go vet ./... is clean — no Go code changed; not run
• Code is gofmt'd — no Go code changed
• Added or updated tests for the change — new Sidebar flyout regression test
• Updated user-facing docs under src/ if behavior changes
• Updated ROADMAP.md if this closes a roadmap item — not a roadmap item
• No secrets, tokens, or webhook URLs introduced in source / YAML
• No new third-party dependencies