* docs: Better document DMS volumes
* docs: Remove any mention of `ONE_DIR` ENV
* chore: Remove `ONE_DIR` ENV from scripts
Only `ONE_DIR=0` has any effect. As the actual feature is now dependent upon the `/var/mail-state` location existing.
It is advised not mounting anything there instead if wanting to avoid runtime state consolidation.
* docs: Adjust link ref convention
This is more search friendly / organized to find references to all DMS volumes.
* lint: Ensure final newline is present
VSCode by default excludes this if the last line rendered is removed (rendered as a separate blank line).
A separate setting can enforce adding the final newline upon save regardless.
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