docs: restructure Getting Started around three explicit phases (#20283)

## Summary

Restructures the apps Getting Started doc around the three things a
developer actually has to do, so the mental model is visible upfront and
discoverable when something goes wrong.

**Why this matters:** the previous flow read as one continuous list of
bash commands and prompts, which made it easy to miss that scaffolding,
running a Twenty server, and live-syncing changes are three separate
concepts. When the user hits a failure (Docker not running, server not
up, auth not authorized), they have no mental map for which step they're
in — so they end up retrying `yarn twenty dev`, which is the only
command they remember.

## What changes


**[getting-started.mdx](https://github.com/twentyhq/twenty/blob/docs/restructure-getting-started-three-phases/packages/twenty-docs/developers/extend/apps/getting-started.mdx):**
- New summary table at the top showing the three-phase arc:

  | Phase | What you do | Tool | Result |
  |---|---|---|---|
| **1. Scaffold** | Generate the app's source code | `npx
create-twenty-app` | A TypeScript project on disk |
| **2. Run a server** | Start a Twenty server to sync into | Docker +
`yarn twenty server` | A running Twenty instance |
| **3. Sync** | Live-sync your code to the server | `yarn twenty dev` |
Your changes appear in the UI |

- Three top-level sections, one per phase, each ending with **"After
this phase: you have X"** so users can self-diagnose where they got
stuck.
- Phase 2 leads with the sentence that was missing before: *"Your app
needs a Twenty server to sync into. The server is a full Twenty instance
— UI, GraphQL API, PostgreSQL — running locally in Docker."* This is the
concept new users were missing.
- Removed the standalone *What are apps?* section — that's what the Core
Concepts page is for. Don't duplicate.
- Tightened wording throughout; same screenshots, same callouts, same
content depth.


**[core-concepts/apps.mdx](https://github.com/twentyhq/twenty/blob/docs/restructure-getting-started-three-phases/packages/twenty-docs/getting-started/core-concepts/apps.mdx):**
- Removed the install snippet (`npx create-twenty-app`, `cd`, `yarn
twenty dev`) — it duplicated Getting Started and the two examples used
different directory names.
- Updated the link card to reflect the new three-phase structure.

## Out of scope (mentioned for context, not done here)

- The "Docker is not running" message rewrite: separate PR
([#20280](#20280)).
- A `yarn twenty start` one-command bootstrap that auto-starts the
server before `dev`. Worth doing — keeping it out of this docs PR.
- Auto-offering to start the server when `yarn twenty dev` finds no
running one. Same.
- An "agent path" doc (single-page, imperative, for AI assistants) —
separate effort.

## Test plan
- [x] `npx nx lint twenty-docs` passes (no new warnings)
- [x] All `<Note>`, `<Warning>`, `<Card>`, image refs preserved
- [ ] Render and click through both pages once merged and previewed

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: FelixMalfait