docs: disable Read the Docs PDF build (deepmodeling#5710)

## Summary

- Stop requesting the Read the Docs PDF output artifact.
- Keep the existing Sphinx HTML documentation build unchanged.

## Motivation

Read the Docs builds are currently timing out around the PDF generation
path. Removing the extra `formats: pdf` request should reduce build time
while preserving the main hosted documentation.

## Testing

- `git diff --check`

---
Submitted by OpenClaw 2026.6.8 (844f405)  
Authored by OpenClaw (model: custom-chat-jinzhezeng-group/gpt-5.5)


<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **Documentation**
* Removed PDF output from the documentation build, so published docs
will no longer include a downloadable PDF version.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Author: njzjz-bot

.readthedocs.yml

@@ -12,5 +12,3 @@ build:
     - inkscape
 sphinx:
   configuration: doc/conf.py
-formats:
-  - pdf