Add guide for replicating tag system

Author: ttuff

TAG_SYSTEM_HOWTO.md

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+# Tag System Setup Guide
+
+This guide explains how to recreate the Data Library's tag system in another repository. The process centers on a small Python script that scans Markdown documentation, builds tag index pages, and updates the navigation for a MkDocs site.
+
+## 1. Prepare your repository
+
+1. Organize your documentation under a top‑level `docs/` directory. Each dataset or topic should live in a nested folder inside `docs/`.
+2. Add a `tags` field to the YAML front matter of each Markdown file or rely on the folder structure to derive tags. Tags should be lowercase and use hyphens for spaces, e.g.
+
+   ```markdown
+   ---
+   title: Example dataset
+   tags: [hazards, remote-sensing]
+   ---
+   ```
+
+3. Ensure Python 3 and the [PyYAML](https://pyyaml.org/) package are available in your environment.
+
+## 2. Copy the tag builder script
+
+1. Copy [`scripts/build_tags.py`](scripts/build_tags.py) from this repository into the `scripts/` directory of your new project.
+2. Adjust the `summit_descriptions` dictionary inside the script if you want special descriptions for particular tags. You can also edit the logic that derives tags from paths in the `derive_tags` function.
+
+## 3. Generate tag pages
+
+Run the script from the repository root:
+
+```bash
+python scripts/build_tags.py
+```
+
+The script will:
+
+- Scan `docs/` for Markdown files and collect their tags.
+- Create `docs/tags.md` with an index of all tags and the pages associated with each tag.
+- Build a `docs/topic/` directory containing tag‑specific pages for the most common tags.
+- Generate a standalone `docs/innovation-summit-2025.md` page when that tag is present.
+- Rewrite `mkdocs.yml` to include the new pages in the site navigation.
+
+Commit the generated files (`docs/tags.md`, any files under `docs/topic/`, `docs/innovation-summit-2025.md`, and the updated `mkdocs.yml`) along with your documentation.
+
+## 4. Update your build process
+
+Include the tag generation step in your documentation build workflow or continuous integration pipeline so that tag pages stay in sync with the Markdown sources.
+
+## 5. Customizing the system
+
+- **Different documentation locations**: If your docs are not in `docs/`, change the `docs_dir` variable at the top of the script.
+- **Alternate navigation**: Modify the `cfg['nav']` block in the script to match your preferred MkDocs navigation structure.
+- **Additional metadata**: Extend the script to read other front‑matter fields if you want more complex tagging logic.
+
+By following these steps, any repository can implement the same automated tag pages used in the Data Library.