Fix formatting in agglomeration_poisson code gallery entry

[xinpinggui]

This pull request addresses the image display issue in the agglomeration_poisson code gallery entry. In the previous version, figures were inserted using inline HTML image tags and layout elements, such as <img>, <div>, and <span>, to control widths, alignment, and side-by-side placement. These figures were displayed correctly on GitHub, but they did not appear properly on the deal.II code gallery page.

To address this issue, this pull request:

• replaces the inline HTML-based image layouts in README.md with standard Markdown image references;
• uses combined PNG figures for image groups that require a fixed side-by-side layout and captions;
• adjusts related formatting in the README;
• updates comment formatting in agglomeration_poisson.cc where appropriate.

These changes are purely related to documentation and formatting. They do not modify the numerical method or the functionality of the example.

[xinpinggui]

Thank you very much for reviewing and merging the PR.