docs: add slice-groups documentation

[Gaic4o]

Background

Add a dedicated documentation page for Slice groups.
This concept was briefly mentioned on the existing Slices and segments page, but lacked a more detailed explanation.

Motivation

The current Slices and segments page only introduces slice groups briefly, without explaining when to use them, what rules apply, or what pitfalls to avoid.

Since slice groups are a recurring topic in the FSD community, this dedicated page helps clarify:

• what a slice group is (and what it is not)
• the core rules behind it
• when introducing it makes sense
• concrete examples in entities and pages
• anti-patterns such as placing shared code at the group level

I placed the page under Reference, after Slices and segments and before Public API, because it felt closer to a structural concept than to a usage guide.

For now, I only added Korean and English versions. If this direction is approved, I’ll prepare translations for the other supported languages as a follow-up.

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[Solant]

I have a feeling that this is a bit verbose for the purpose of slice groups

[Solant]

Let's do a very strict ban on any code reuse instead, so the message is very clear, just like in https://fsd.how/docs/reference/slices-segments/#slice-groups

[Gaic4o]

Yeah, I agree. I’ll rephrase this more strictly. Also, I think if we include the image from the slice groups section in the docs, we can probably shorten or omit some of the explanation I added here.

[Solant]

nit: do we need to remove prefix part from the grouped slices? like

- pages/
  - order/
    - create/

or

- pages/
  - order/
    - order-create/

?

[Gaic4o]

I’ll go with create/ here, since the parent group already provides that context.

[Solant]

This is definitely new; I've never used this pattern for pages 🤔

[Gaic4o]

I wanted to show that slice groups may also be used in the pages layer. I included this as an example, not as a default pattern.

[Solant]

I think slice groups were originally introduced to primarily tackle feature layers (I think in 2.0), so it is strange to ban it like this

[Gaic4o]

I see your point. I agree this should not be framed as a ban. As I mentioned before, grouping in the features layer can be less obvious because a feature often spans multiple entities and flows. So rather than treating it as exceptional, I think it would be better to clarify that slice groups can work in some cases but not in others, depending on whether there is a natural grouping criterion. I’m thinking of reflecting that in the document with examples of cases where grouping is a good fit and where it is not.

[Solant]

I think we need to ban even the idea of doing code share on the slice-group level in the first lines of the document (not literally, but you get the idea).

[Gaic4o]

I see what you mean. I’ll make that explicit in the opening section, and then I think the later explanation can be removed.

[Solant]

Is it the question regarding absolute vs. relative imports?

[Gaic4o]

I meant that part as a note about absolute vs. relative imports, but I agree it doesn’t really belong here. I’ll remove it. Thanks for catching that.