Add comprehensive README for src/journeys (#58876)

Author: heiskr

src/journeys/README.md

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+# Journeys
+
+The **journeys** subject provides guided learning experiences (called "tracks") that help users navigate through a sequence of related documentation articles. Tracks appear on special landing pages and provide contextual navigation to move through articles in a structured learning path.
+
+## Purpose & Scope
+
+This subject is responsible for:
+- Rendering journey landing pages that display multiple learning tracks
+- Providing prev/next navigation within journey track articles
+- Resolving journey context based on the current article path
+- Rendering Liquid templates in journey metadata (titles, descriptions, guide paths)
+
+Journey tracks are defined in article frontmatter using the `journeyTracks` field on pages with `layout: journey-landing`.
+
+## Architecture & Key Assets
+
+```
+src/journeys/
+├── components/
+│   ├── JourneyTrackCard.tsx    # Card showing journey progress with next/prev links
+│   ├── JourneyTrackNav.tsx     # Navigation bar for prev/next articles in a track
+│   └── index.ts                # Component exports
+├── lib/
+│   ├── get-link-data.ts        # Fetches title and href data for journey guide links
+│   └── journey-path-resolver.ts # Core logic: resolves journey context and tracks
+├── middleware/
+│   └── journey-track.ts        # Express middleware that attaches journey data to requests
+└── tests/
+    └── journey-path-resolver.ts # Unit tests for journey resolution logic
+```
+
+
+
+## Setup & Usage
+
+### Prerequisites
+- Journey landing pages must have `layout: journey-landing` in their frontmatter
+- Journey tracks are defined in the `journeyTracks` frontmatter field (see example below)
+
+### Running tests
+```bash
+npm run test -- src/journeys/tests
+```
+
+### Example frontmatter for journey landing page
+```yaml
+---
+title: Enterprise onboarding
+layout: journey-landing
+journeyTracks:
+  - id: 'getting_started'
+    title: 'Getting started with {% data variables.product.prodname_ghe_cloud %}'
+    description: 'Master the fundamentals and get started with a trial.'
+    guides:
+      - '/enterprise-onboarding/choose-an-enterprise-type'
+      - '/enterprise-onboarding/setting-up-a-trial'
+      - '/enterprise-onboarding/adding-users'
+---
+```
+
+## Data & External Dependencies
+
+### Data inputs
+- **Content frontmatter**: `journeyTracks` field on landing pages defines track structure
+- **Article metadata**: Article titles and paths are resolved via `get-link-data.ts`
+- **Liquid variables**: Track titles, descriptions, and guide paths support Liquid templating
+
+### Dependencies
+- **`@/content-render`**: Used to render Liquid templates in journey metadata
+- **`@/frame/lib/path-utils`**: Normalizes paths for consistent matching
+- **`@/versions`**: Checks version compatibility between journey pages and articles
+- **`@/languages`**: Executes rendering with fallback for internationalization
+- **`@/landings`**: Journey components are consumed by landing page layouts
+
+### Data outputs
+- **`req.context.currentJourneyTrack`**: Journey context object with track info and prev/next links
+- **`req.context.page.resolvedJourneyTracks`**: Array of resolved track data for landing pages
+
+## Current State & Next Steps
+
+### Known limitations
+- Journey tracks currently inherit version constraints from their landing page
+- Path normalization logic must stay synchronized with other path-handling middleware
+- Journey context resolution has some performance overhead due to iterating all pages
+- Currently only support a particular page belonging to a single journey track/step - we won't show nav components for all the journeys an article belongs to
+
+Continued work to expand and add more journey tracks.