dedicated visualizer docs page and README diagram block

[joshua-temple]

Split from #307.

This issue delivers a feature plus its documentation: a cascade graph --update flag that
keeps a committed diagram current, and the docs that explain the graph command surface and
the marker-block workflow.

Feature

Add a cascade graph --update <file> flag that rewrites a fenced mermaid code block located
between sentinel comment markers in a target file. It is opt-in and non-destructive:

• It only replaces the content between the sentinel markers, leaving the rest of the file
  byte-identical.
• It is a clear no-op with an informative message if the markers are absent.
• Re-running it is idempotent: a second run against an already-current file produces no
  change.

Docs

• A dedicated docs page under docs/src/content/docs covering the graph granularities
  (jobs, stages, env, cross-repo) and the --theme flag, with a rendered mermaid example.
• A README marker-block workflow demonstrating how to commit a rendered diagram that stays
  current via --update.

cli-reference already documents the graph command (landed in #421). This issue adds the
--update feature, the standalone docs page, and the README slice, not the command
reference.

Files and areas

• internal/graph/command.go - the new --update flag and the marker-rewrite logic.
• The new docs page under docs/src/content/docs.
• README.md - the marker-block workflow.

Acceptance

• Unit tests for the marker rewrite: idempotent re-run, absent-marker no-op, and surrounding
  bytes preserved.
• An e2e scenario under the repo's CLI-surface standard exercising cascade graph --update.
• A docs page documents the granularities and --theme, with a rendered mermaid example.
• The README marker-block workflow is documented so a user can commit and refresh a diagram.
• The docs site builds.

[joshua-temple]

Deferred with the visualizer epic #300 (see its rationale). Not rejected; revisit if real demand for a state-overlay or what-if visualization appears.