Explain that auto-upgrade follows every published stack revision

[csandanov]

The stack docs said that only automatic stack updates auto-upgrade app environments. They also said that manual stack updates, manual syncs and published drafts never move app environments forward. That's not how auto-upgrade works. An app environment with auto-upgrade enabled moves to the stack's latest published revision during its upgrade time window, however that revision was published. Automatic stack updates only start it sooner.

This updates the three pages that describe it:

• Application stack → Auto-upgrade:
  • Auto-upgrade follows every published revision. Automatic stack updates start it right away, and Wodby also regularly checks for environments with auto-upgrade enabled that still run an older revision.
  • To control a rollout yourself, disable auto-upgrade for those environments or keep the change in a draft. Unpublished drafts are never applied.
• Stacks overview and Stack updates: publishing a revision moves app environments with auto-upgrade enabled during their upgrade time window. Other environments keep their revision until they are upgraded.

This matches current product behavior and can be merged now.

Validation

• Links point to the existing apps/stack.md#auto-upgrade anchor, which other pages already use.