Generated site contains a lot of invalid pages

While testing some accessibility linting tools for #396, I noticed that `sitemap.xml` contains lots of url pointing to the default zola html for non-rendered section/page (see [here](https://bevyengine.org/assets/assets/ai/big-brain/) for example).

It seems this is due to how the `generate_*` tooling generates the content, as we end up with lots of markdown files that actually do not correspond to a section/page in `content` (they are used to build lists [like here](https://bevyengine.org/assets/) instead).

A possible solution that I tested would be to add `render = false` to the generated sections front matter, and manually exclude the generated pages from the sitemap. There is no equivalent to `render = false` for pages, so this need to be done with an extra attribute and a custom sitemap template, but this does not completely solves the problem as the invalid html is still built. The simplest way to fix that would probably to have the `render` attribute be implemented by Zola for pages, otherwise we would need to change how list pages are generated to avoid going through some intermediate markdown.

Start by tracing the generate_* tooling and the generated markdown front matter, then inspect sitemap.xml and the custom sitemap template. Done means generated list-only content no longer produces invalid default pages or sitemap URLs; the issue leaves the implementation choice open between Zola support and changing content generation.

Written by the indexing model from the issue text.