docs(plans): explain test plans as a concept, not a yaml file

[philipph-askui]

What changed for the reader

The Test plans page currently opens with "A plan is a yaml file in `plans/`" and a yaml block, so it reads as documentation of a file format. A tester meets the storage detail before learning what the feature is.

This rewrite leads with the concept and the workflow instead:

• Intro now says what a test plan is and what you do with it: a saved selection of tests and the device they run on, run with one click or on a schedule, with nothing inferring the selection.
• Sections follow the journey: Build a plan (check tests + pick a device), Run it with one click, Put it on a schedule, Share it through Git.
• The yaml example is demoted to a short "Under the hood: the plan file" section, framed as "you never edit it directly."
• Older prose plans kept.

No behaviour claims changed. Everything here already shipped:

• App: askui/integrated-task-platform#180 (Test Plans: structured `plans/.yaml`)
• Docs companion that first added the page: Test plans: yaml files with explicit selection and devices #15

Verification

• `npm run types:check` — green (MDX compiles, route types generate, `tsc --noEmit` clean).
• All internal links resolve to real pages; the `#if-it-cant-start-right-away` anchor exists on the Running tests index. No #anchor elsewhere targets this page, so the heading restructure breaks nothing.
• House style: no em dashes / interpuncts in prose; numbered click path for building a plan; bold-label bullet walkthroughs.

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