Add units flow page to docs

Author: teutoburg

docs/source/faqs/scopesim_units_flow.md

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+# Approximate sketch of "units flow" in ScopeSim
+
+Effects that change units are shown in hexagonal boxes.
+Rectangular boxes are basically `ImageHDU` wrappers.
+Parallelogram boxes are lists of other entities.
+
+```mermaid
+%%{init: {"flowchart": {"htmlLabels": false}} }%%
+flowchart TB
+    Source(["Source [PHOTLAM(/arcsec2)]"])
+    FOV1[/"FOV.fields"/]
+    FOV2["FOV.hdu [ph/s]"]
+    FOV3[/"FOV.fields"/]
+    FOV4["FOV.hdu [ph/s/um/arcsec2]"]
+    FOV5["FOV.hdu [ph/s]"]
+    SPT{{"`SpectralTrace:
+          dispersion, sum(sky)`"}}
+    IMP["ImagePlane [ph/s]"]
+    Det1["Detector [ph]"]
+    QECurve{{"QECurve: ph -> e-"}}
+    Det2["Detector [e-]"]
+    ADC{{"ADConversion: e- -> ADU"}}
+    Det3["Detector [ADU]"]
+    Output(["Output [ADU]"])
+
+    Source-- extract -->FOV1
+    subgraph Imaging
+        FOV1-- "sum(wave), *area" -->FOV2
+    end
+    Source-- extract -->FOV3
+    subgraph Spectroscopy
+        FOV3-- *area -->FOV4
+        FOV4-->SPT
+        SPT-->FOV5
+    end
+    FOV2-- project -->IMP
+    FOV5-- project -->IMP
+    IMP-- "readout: sum(time)" -->Det1
+    Det1-->QECurve
+    QECurve-->Det2
+    Det2-->ADC
+    ADC-->Det3
+    Det3-->Output
+```