Adding summary_overview.md page as a default. I've considered generating this with the plugin, but since (1) the generated content is consistent throughout libraries, and (2) the overhead on creating the files and working with the rest of the markdown files were taking up a bit too much engineering time, I've opted to creating this as the base file using templating instead.

See design: https://docs.google.com/document/d/1FmAOv9ald2W2Set8jPzN-gwQoE992LCWSu40KBVON28/edit?resourcekey=0-JLhux0549oJustMl46l3zA&tab=t.0#heading=h.spykynl1p438. The design allows individual library maintainers to also add additional content into the overview page as needed, see https://cloud.google.com/python/docs/reference/bigframes/latest/summary_overview as an example.

The comment is added (in [...]: # format) to help guide authors in the future if they want to add additional content into the overview pages. Verified that it does not convert comment into any HTML comment, and is fully stripped in the actual content.

With this change, we'll be able to redirect folks to the summary_overview page instead of looping back to the same entry page.

Sphinx support for overview pages: googleapis/sphinx-docfx-yaml#365

Fixes b/263399076.