Add README for multi-source docs example

multi-source/README.md

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+# Multi-Source Docs
+
+A working example of [Fern's multi-source documentation](https://buildwithfern.com/learn/docs/getting-started/multi-source-docs) — six independent projects publishing to one domain, each with its own `fern/` directory and deploy lifecycle.
+
+**Live site:** [multi-source.docs.buildwithfern.com](https://multi-source.docs.buildwithfern.com)
+
+![Homepage](./assets/homepage.png)
+
+## Project structure
+
+```
+multi-source/
+├── homepage/          → /                     (root landing page + theme source)
+├── seeds/             → /seeds                (team hub with nested children)
+├── seeds-sunflower/   → /seeds/sunflower      (sub-team site)
+├── seeds-tomato/      → /seeds/tomato         (sub-team site)
+├── greenhouses/       → /greenhouses          (standalone team site)
+└── nursery/           → /nursery              (standalone team site)
+```
+
+Each folder contains a `fern/` directory with its own `docs.yml`, `fern.config.json`, and pages.
+
+## Global theme
+
+All six sites share a `global-theme` called `plantstore-theme`. It's defined once in the homepage project and uploaded to Fern's registry, so every sub-site gets consistent branding with zero duplication.
+
+### What the theme provides
+
+- **Colors** — light tan background, dark forest green accents, with dark mode variants
+- **CSS** — card hover effects, sidebar typography, heading letter-spacing
+- **Logo** — shared leaf icon with "Multi-Source Docs" text
+
+### theme.yml
+
+```yaml
+colors:
+  accent-primary:
+    dark: '#70E155'
+    light: '#1B4332'
+  background:
+    dark: '#1A1A17'
+    light: '#F5F0E8'
+  border:
+    dark: '#3A3A30'
+    light: '#D4C9B5'
+  sidebar-background:
+    dark: '#1F1F1B'
+    light: '#F0EBE1'
+  header-background:
+    dark: '#1A1A17'
+    light: '#F5F0E8'
+  card-background:
+    dark: '#252520'
+    light: '#EDE8DC'
+css: ./assets/styles.css
+logo:
+  href: /
+  dark: ./assets/logo-dark.svg
+  light: ./assets/logo-light.svg
+  height: 28
+  right-text: Multi-Source Docs
+```
+
+### Uploading the theme
+
+```bash
+# Export theme-eligible fields from docs.yml into a theme/ directory
+fern docs theme export
+
+# Upload to the registry under a name your org can reference
+fern docs theme upload --name plantstore-theme
+```
+
+Every other site references it with a single line in their `docs.yml`:
+
+```yaml
+global-theme: plantstore-theme
+```
+
+## Sub-site docs.yml
+
+Each sub-site's configuration is minimal — the theme handles branding, so teams only define their instance URL, navbar links, and page navigation.
+
+![Sub-site](./assets/subsite.png)
+
+Here's the complete `docs.yml` for the Sunflower sub-site:
+
+```yaml
+global-theme: plantstore-theme
+
+instances:
+  - url: multi-source.docs.buildwithfern.com/seeds/sunflower
+    multi-source: true
+
+title: Sunflower
+
+navbar-links:
+  - type: minimal
+    text: Home
+    href: /
+  - type: minimal
+    text: Seeds
+    href: /seeds
+
+navigation:
+  - section: Sub-site config
+    contents:
+      - page: Overview
+        path: ./pages/overview.mdx
+      - page: Per-site navbar
+        path: ./pages/planting.mdx
+```
+
+The key setting is `multi-source: true` on the instance — this tells Fern to publish to a sub-path without overwriting other sites on the same domain.
+
+## Publishing
+
+Each site publishes independently:
+
+```bash
+cd multi-source/seeds-sunflower
+FERN_TOKEN=$FERN_TOKEN fern generate --docs
+```
+
+Sites can be published in any order, at any time, without affecting each other.
+
+## Features demonstrated
+
+| Feature | Where to see it |
+| --- | --- |
+| Multi-source publishing | Every sub-path is a separate project |
+| Nested sub-children | `/seeds` → `/seeds/sunflower`, `/seeds/tomato` |
+| Global theme | Shared colors, CSS, and logo via `plantstore-theme` |
+| Per-site navbar | Each site defines its own header links |
+| Hierarchical search | Search from `/seeds` includes child paths |
+| Independent deploys | Teams publish without coordinating |