docs: add canonical pipeline guide (#937)

[CypherPoet]

Closes #937.

What

Adds docs/guides/pipeline.mdx -- a dedicated guide for the 7-step
production pipeline (Capture → Design → Script → Storyboard → VO +
Timing → Build → Validate). Cross-references it from
website-to-video, quickstart, prompting, and launch-videos.

Why

Previously the pipeline was documented as a small table inside the
website-to-video guide. That under-sold it: The structure (and the
DESIGN.md / SCRIPT.md / STORYBOARD.md artifacts) is useful for
any multi-beat Hyperframes project, not just website captures, and
it's a natural orientation read for anyone learning the framework.

How

• New page expands each step with: what the artifact is, what's in it,
  what the gate is for moving on, and how to iterate without re-running
  the whole pipeline. Includes a project-layout diagram.
• Registered first in the Guides section in docs.json. It's
  orientation for the rest of the Guides, followed by website-to-video
  (the headline use case) and then the technique-specific guides.
• website-to-video keeps a brief overview table and links to the new
  guide; deeper detail no longer duplicated.
• Cross-links added wherever STORYBOARD.md / DESIGN.md were already
  referenced (launch-videos) or where the /website-to-hyperframes
  skill is named (prompting).

Test plan

• mintlify dev: new page renders, all cross-links resolve, nav
  order matches the intended pipeline → website-to-video → ….
• Verified the website-to-video page still reads coherently with
  the trimmed pipeline section.
• No code changes; no unit tests applicable.