Clarify the use of svelte-kit preview and adapters

[stephane-vanraes]

This PR simple adds an extra comment to svelte-kit preview noting that serving the final app is dependent on the adapter used and refers the developers to the documentation of the adapter to learn how to actually serve their app in production.

Reason for this change is that is a recurring question, the documentation tells that you should not use the preview command but does little in explaining where to get the information on how to do it.

[benmccann]

the new sentence seems out of place to me because running your app in production with the adapter is unrelated to svelte-kit preview. it seems like it might belong in the svelte-kit build section or something instead

[stephane-vanraes]

I placed it here because the preceding sentence mentions it is not for serving your app leaving several users wonder what the correct command is for serving the app.
Ideally a comment like this (I can have another look later) would be somehow integrated in the sections about adapters themselves.

I feel the documentation is lacking here as this is a question that comes back every other day on the Discord in some form or another, but clearly some more discussion can be had here.

I would propose to move the wording indeed to svelte-kit build with something along the lines of

After building you can run your app, see the chosen adapter for specific instructions on how to do this.

In preview remove the new lines, but add a link to the adapter section.
And in the adapter section another note that you should check the documentation of the adapter for serving instructions.

[jthegedus]

I think the following line in the preview docs still needs to be more clear:

-This is intended for testing the production build locally, **not for serving your app**, for which you should always use an adapter
+This is intended for testing the production build locally, **not for serving your app in the adapter target environment locally** (for locally testing a prod build with the adapter output see the adapter docs).

Or something of the sort. The distinctions I see that need to be made are:

1. dev: svelte-kit dev
2. build prod (used for all subsequent steps): svelte-kit build
3. test prod build locally without adapter environment: svelte-kit preview
4. test prod build locally with adapter environment: see adapter docs
5. deploy prod build to adapter environment: see adapter docs

with steps 3 and 4 being the confusing pieces. 3 relates to preview so should explicitly mention not running in the adapters env, even though you just built using the adapter. 4 relates to build so should be there.

The docs don't make these 5 steps clear in a concise manner. You have to read a lot of documentation to gather the information on these 5 steps.