[Feature Request]: Allow docs-related PR previews to select between edge and release version of docs-builder

### Summary

Today PR previews build with the edge version of docs-builder. This means that:

- A PR can be merged but merging it could break the build if it uses things only working with edge
- PR previews can show you a different state than what your PR would look like in prod

I feel strongly that PR previews should show you what you get if you merge the PR now.

### Additional Context

Discussion and examples: https://github.com/elastic/docs-builder/pull/3378 and associated Slack thread https://elastic.slack.com/archives/C05PP2LEC1X/p1779781826970869 (elastic-internal).

### How important is this feature to you?

"Important"

### Proposed Solution

Default to the released version and offer an "opt-in" way to use edge as I understand it can be useful for testing. For example, based on the presence of a "docsPreview:edge" label.

[View original Slack conversation](https://elastic.slack.com/archives/C05PP2LEC1X/p1779781826970869?thread_ts=1779781826.970869&cid=C05PP2LEC1X)