docs(rendering): clarify usage with render_greetings.py

- fixture setup, exclusive flags, and slide preview guidance

Signed-off-by: Piotr Król <piotr.krol@3mdeb.com>

docs/rendering.md

@@ -2,34 +2,64 @@
 
 Prerequisites
 
-- Python 3
+- Python 3.10+
 - pip install -r requirements.txt (adds Jinja2 and pytest if not present)
 
 Obtaining the pretalx export (example for dug_10):
 
 ```bash
+# Ensure fixtures directory exists for the schedule export
+mkdir -p fixtures
+
+# Download and save the schedule XML into fixtures
 curl -o fixtures/dug_10_schedule.xml "https://cfp.3mdeb.com/developers-vpub-0xf-2025/schedule/export/schedule.xml"
 ```
 
-Render into a temporary directory (won't overwrite existing pages by default):
+Render into a target output
+
+- Render from a schedule XML to an output directory (won't overwrite existing
+pages by default):
 
 ```bash
 python3 scripts/render_greetings.py --schedule fixtures/dug_10_schedule.xml --dug-num 10 --output /tmp/outdir
 ```
 
-Compare generated file with authoritative repository file:
+- Render from a DUG number (pretalx URL inferred):
+
+```bash
+python3 scripts/render_greetings.py --dug-num 10 --output /tmp/outdir
+```
+
+- If you want to overwrite the repository page directly, provide --force (and
+ensure the output path matches the repo file):
+
+```bash
+python3 scripts/render_greetings.py --dug-num 10 --output pages/dug_10/1-greetings-agenda.md --force
+```
+
+Compare generated vs repository
 
 ```bash
 diff -u pages/dug_10/1-greetings-agenda.md /tmp/outdir/1-greetings-agenda.md
 ```
 
-Preview locally (project helper):
+Preview locally
+
+- The repository does not include a dedicated local-preview script. To preview,
+view the generated Markdown in a Markdown viewer/editor, or render slides with
+the Slidev-based template if you want slide previews:
 
 ```bash
-./scripts/local-preview.sh pages/dug_10/1-greetings-agenda.md
+git submodule update --init --checkout
+slidev-template/scripts/render-slides.sh pages/dug_10/1-greetings-agenda.md
 ```
 
+- Open the resulting slides in your browser as instructed by Slidev.
+
 Troubleshooting
 
-- If the generated file misses slides links, check that the pretalx description contains a bullet with `[slides](https://...)`.
-- If banner path differs, adjust `--banner-width` or edit `templates/1-greetings-agenda.j2` to change the image path.
+- If the generated file misses slides links, check that the Pretalx description
+contains a bullet with a slides URL, and/or edit
+`templates/1-greetings-agenda.j2` to render slides (ev.slides).
+- If banner path differs, adjust `--banner-width` or edit the template path
+accordingly.