Doc: Make zipped HTML download contain individual pages, not a single page

[dbaston]

Feature description

As noted by @jidanni in #12093 (comment) , the zipped HTML download contains all of the documentation in a single HTML page.

This seems be an intended behavior for ReadTheDocs (https://stackoverflow.com/questions/32771287/download-html-as-separate-pages). Perhaps we can work around it somehow.

Additional context

No response

[jidanni]

The funny thing is that you offer two zips there on gdal.org, the multiple pages one that I want, and the single page one that I don't want.

The problem is the multiple pages one is old, and the single pages one is new.

I'm hoping for a multiple pages one that is new. (Reflects the bleeding edge version.)

As far as the web addresses of each, etc. I believe this bug report is getting confusing enough already...

[jratike80]

The single page behavior is also documented in https://docs.readthedocs.com/platform/stable/downloadable-documentation.html.