Refactor autogenerated sidebar groups

[HiDeoo]

Description

• This PR adds a new feature which has been discussed and approved here.

• Closes Groupless autogenarated sidebar links #3336

This PR removes the notion of "autogenerated sidebar groups" in favor of simply using "autogenerated sidebar links" which can be used in regular sidebar groups. As discussed in #3336, this simplifies the mental model around groups and also allows for use cases that were not possible before, such as having a sidebar entirely made of autogenerated links for a specific directory without having it nested inside a group.

For users, the gist of the change is:

{
    label: 'My group',
-   autogenerate: { directory: 'some-dir' },
+   items: [{ autogenerate: { directory: 'some-dir' } }],
}

Such refactor removes the 1:1 mapping that was previously available between an autogenerated sidebar group in the Starlight configuration and such autogenerated group in the Starlight route data. To address such change, the Starlight route data has been updated and many solutions are possible to address this, e.g. having multiple types (link, autolink, group, and autogroup). This PR has gone through multiple variations of such approach, and the current implementation does not introduce new types but rather adds an autogenerated property set to to true for links and subgroups that were autogenerated. Of course, other approaches are possible and feedback is welcome.

To make sure such change does not impact plugins with previous features that would no longer be possible due to the changes, I rewrote locally the starlight-auto-sidebar plugin, which was heavily relying on that 1:1 mapping and to my knowledge, may be one of the "largest" plugins tweaking sidebar route data for autogenerated groups with a large enough set of tests. I managed to rewrite it successfully while using the Starlight version from this PR and achieved the same results as before.

Note: I also addded a few tests for some behaviors that were not covered before and were impacted by this change, e.g. the collapsed state of autogenerated subgroups.

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Locale	File	Note
en	guides/sidebar.mdx	Source changed, localizations will be marked as outdated.
en	reference/frontmatter.md	Source changed, localizations will be marked as outdated.

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[HiDeoo]

I need to investigate why this error was hoisted (I can guess why, but need to know the specific reason) and see if we can move it back down at the group item level without losing some other type behaviors we expect.