fix: broken image links in deep research notebooks

[ali90h]

Summary

Fixes broken image links in the two Deep‑Research notebooks (introduction_to_deep_research_api.ipynb and introduction_to_deep_research_api_agents.ipynb).
The relative paths (../images/*.png) were replaced with raw GitHub URLs so the images render correctly on GitHub and the cookbook website.

Motivation

Readers of the Deep‑Research notebooks currently see missing‑image icons, which makes the tutorial harder to follow.
Updating the links restores the diagrams, improving clarity and the overall quality of the documentation.

---

For new content

Not applicable: this PR only patches existing notebooks, it does not introduce new articles or examples, so no registry.yaml entry is needed.

When contributing new content, read through our contribution guidelines, and mark the following action items as completed:

• I have added a new entry in registry.yaml (and, optionally, in authors.yaml) so that my content renders on the cookbook website.
• I have conducted a self-review of my content based on the contribution guidelines:
  • Relevance: This content is related to building with OpenAI technologies and is useful to others.
  • Uniqueness: I have searched for related examples in the OpenAI Cookbook, and verified that my content offers new insights or unique information compared to existing documentation.
  • Spelling and Grammar: I have checked for spelling or grammatical mistakes.
  • Clarity: I have done a final read-through and verified that my submission is well-organized and easy to understand.
  • Correctness: The information I include is correct and all of my code executes successfully.
  • Completeness: I have explained everything fully, including all necessary references and citations.

We will rate each of these areas on a scale from 1 to 4, and will only accept contributions that score 3 or higher on all areas. Refer to our contribution guidelines for more details.