Document the dashboard, the artifacts command and what moved with them

Covers this round of CLI work: the serve dashboard and its keys, the refusal
to start a second engine for one installation, --plain, --gpus and the
declared-not-detected card count, `fluksio artifacts`, the age column,
`fluksio.logger` inside a node, the server extra, per-directory study module
names, and an export whose columns no longer move with the selection.

docs/reference/configuration.md

@@ -134,10 +134,11 @@ warning into a refusal to start.
 | `ARTIFACT_GC_GRACE_S` | `3600` | how long a freshly written artifact is spared, whatever refers to it |
 
 The three concurrency limits are also flags on `fluksio serve` — `--max-workers`,
-`--max-cascades`, `--max-runs` — which outrank the file, and the engine says
-which numbers it started with in its first lines. Each is a pool size, so each
-must be at least 1: a `0` is refused at startup by name rather than read as the
-default. Leave one empty (or unset) to get the default.
+`--max-cascades`, `--max-runs` — as is the card count, `--gpus`. The flags
+outrank the file, and the engine says which numbers it started with in its
+first lines. Each pool size must be at least 1 and the card count at least 0:
+a number below that is refused as a flag error naming it, rather than read as
+the default. Leave one empty (or unset) to get the default.
 
 An artifact is referred to by a run that recorded it or by a message currently
 holding it; anything else is what a camera published four hours ago, and the