docs: the codegraph ui section, changelog entry and telemetry posture (CG-47)

A new user can now reach the viewer from the README alone: a "Read your
graph in the browser" section with a screenshot re-shot from the real
build, a step 5 in Get Started, a CLI Reference row, and the same
content as a docs-site guide.

- README: new section (what the three columns are, the options, the
  privacy posture), Contents entry, Get Started step 5, CLI row.
  Screenshot at assets/codegraph-ui-symbol-view.png, version-tagged ?v=1.
- CHANGELOG: an [Unreleased] New Features entry in the user-facing voice.
- codegraph help ui: mentions the `web` alias, says what the screen shows,
  and states that nothing is sent anywhere.
- TELEMETRY.md: the viewer has no telemetry of its own and makes no
  outbound connections; the only thing recorded is the command name in
  the daily rollup, which every off-switch already suppresses.
- site/: guides/viewer.md + sidebar entry, a `ui` section in the CLI
  reference, and a link from Next Steps.

site/astro.config.mjs

@@ -74,6 +74,7 @@ export default defineConfig({
 					label: 'Guides',
 					items: [
 						{ label: 'Indexing a Project', slug: 'guides/indexing' },
+						{ label: 'Reading Your Graph in the Browser', slug: 'guides/viewer' },
 						{ label: 'Framework Routes', slug: 'guides/framework-routes' },
 						{ label: 'Affected Tests in CI', slug: 'guides/affected-tests' },
 					],