This PR is mainly intended to simplify the work in #28084 towards switching to pydata-sphinx-theme. I made special adjustments so that it can target main without relying on the setup in #28132.

This PR simplifies the structure of the about and testimonials/testimonials pages. In short, it does the following:

• Used a similar strategy when implementing dropdowns, so that we do not need to repeat the .. raw:: html blocks over and over again. More importantly, the class names are no longer distributed all over the document so it is easier to change class name or styles.
• Removed the hardcoded image widths and replaced by some css styling, i.e., limiting maximum width to 50% of the container (i.e., ~25% of the article width) and maximum height to 50px (for testimonials/testimonials) and 100px (for about).

There are some other minor changes, such that if undesired, please feel free to let me know so that I can revert:

• I removed the "history" subsection title on top of the about page, the reason being that two adjacent headings do not look that well (too much space in between) in pydata-sphinx-theme.
• I made "authors", "documentation team", etc. one level lower, i.e., under the "governance" section.