docs: document PAGES.md template format for agentic-wiki-writer (#262)

* Initial plan

* docs: document PAGES.md template format in agentic-wiki-writer docs

Co-authored-by: mrjf <180956+mrjf@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: mrjf <180956+mrjf@users.noreply.github.com>

Author: Copilot

docs/agentic-wiki-writer.md

@@ -55,6 +55,171 @@ The workflow triggers automatically on every merged PR to the default branch. Yo
 
 After editing the workflow file, run `gh aw compile` to update the compiled workflow and commit all changes to the default branch.
 
+## PAGES.md Format
+
+The `PAGES.md` file at `.github/agentic-wiki/PAGES.md` is the single source of truth for your wiki structure. It is generated automatically on the first run and saved as a PR for you to review. After that, you can edit it freely to customize your documentation structure. You can also supply a `PAGES.md` yourself before the first run to pre-define the structure.
+
+### Header Hierarchy
+
+| Level | Purpose | Output |
+|-------|---------|--------|
+| H1 (`#`) | Top-level page | Separate `.md` file, top-level sidebar entry |
+| H2 (`##`) | Nested page | Separate `.md` file, indented under parent in sidebar |
+| H3 (`###`) | Deeply nested page | Separate `.md` file, further indented in sidebar |
+| H4+ (`####`) | Section within page | H2+ header in rendered page, not in sidebar nav |
+| H4+ with `+` (`####+`) | Sidebar section | H2+ header in page, included in sidebar nav |
+
+### Query Templates
+
+Use `*{ query }*` syntax to mark content that should be AI-generated:
+
+```markdown
+# Home
+
+*{ Provide an overview of this project }*
+
+## Architecture
+
+*{ Describe the system architecture and key design decisions }*
+```
+
+Static text is preserved as-is:
+
+```markdown
+# Getting Started
+
+This guide will help you set up the project.
+
+*{ List the installation steps }*
+
+For more help, see the troubleshooting section.
+```
+
+### Sidebar Sections
+
+By default, H4+ headers become sections within a page but don't appear in the sidebar. Add `+` after the hashes to include them in sidebar navigation:
+
+```markdown
+# API Reference
+
+*{ Overview of the API }*
+
+####+ Authentication
+*{ Describe auth flow }*
+
+####+ Rate Limits
+*{ Describe rate limiting }*
+
+#### Internal Details
+*{ Implementation details - not shown in sidebar }*
+```
+
+This generates:
+- Sidebar: `API Reference` with nested links to `#Authentication` and `#Rate Limits`
+- Page: All three sections rendered as H2 headers
+
+### Header Normalization
+
+Sections (H4–H6) are normalized when rendered to individual pages:
+
+| In PAGES.md | In rendered page |
+|-------------|-----------------|
+| `####` / `####+` | `##` |
+| `#####` | `###` |
+| `######` | `####` |
+
+Every page starts with an implicit H1 (the page title, rendered by GitHub from the filename). Sections start at H2.
+
+### Slug Generation
+
+Page and section slugs are generated from titles:
+- Spaces → hyphens
+- Special characters removed (apostrophes, parentheses, question marks, etc.)
+- Multiple hyphens collapsed
+
+| Title | Slug |
+|-------|------|
+| `Getting Started` | `Getting-Started` |
+| `What's New?` | `Whats-New` |
+| `API Reference (v2)` | `API-Reference-v2` |
+
+### Complete Example
+
+```markdown
+# Home
+
+Welcome to the project documentation.
+
+*{ Provide a brief overview of the project }*
+
+# Architecture
+
+*{ Describe the high-level architecture }*
+
+## Frontend
+
+*{ Describe the frontend stack }*
+
+### Components
+
+*{ List major React components }*
+
+####+ State Management
+*{ Explain how state is managed }*
+
+####+ Routing
+*{ Describe the routing setup }*
+
+## Backend
+
+*{ Describe the backend architecture }*
+
+### API
+
+*{ Document the REST API }*
+
+####+ Endpoints
+*{ List all endpoints }*
+
+# Getting Started
+
+*{ Write a getting started guide }*
+
+#### Prerequisites
+*{ List prerequisites }*
+
+#### Installation
+*{ Installation steps }*
+```
+
+**Output files:**
+
+| File | Content |
+|------|---------|
+| `Home.md` | H1 title + overview content |
+| `Architecture.md` | H1 title + architecture content |
+| `Frontend.md` | H1 title + frontend content + State Management (H2) + Routing (H2) sections |
+| `Components.md` | H1 title + components content |
+| `Backend.md` | H1 title + backend content |
+| `API.md` | H1 title + API content + Endpoints (H2) section |
+| `Getting-Started.md` | H1 title + guide content + Prerequisites (H2) + Installation (H2) sections |
+| `_Sidebar.md` | Auto-generated navigation |
+
+**Generated sidebar:**
+
+```markdown
+- [[Home|Home]]
+- [[Architecture|Architecture]]
+  - [[Frontend|Frontend]]
+    - [[State Management|Frontend#State-Management]]
+    - [[Routing|Frontend#Routing]]
+    - [[Components|Components]]
+  - [[Backend|Backend]]
+    - [[API|API]]
+      - [[Endpoints|API#Endpoints]]
+- [[Getting Started|Getting-Started]]
+```
+
 ## Learn More
 
 - [Agentic Wiki Writer source workflow](https://github.com/githubnext/agentics/blob/main/workflows/agentic-wiki-writer.md)