[webdoc109] webdoc: regularize Offspec and GISAS sim ()

hugo/content/py/fitting/instrument-description/index.md

hugo/content/py/instr/det/_index.md

hugo/content/py/instr/det/offspec.md

hugo/content/py/instr/det/rectangular-detector/index.md

...y/instr/det/rectangular-detector/rectangular_detector.png

.../rectangular-detector/rectangular_detector_directbeam.png

.../det/rectangular-detector/rectangular_detector_genpos.png

...ctangular-detector/rectangular_detector_reflectedbeam.png

...t/rectangular-detector/rectangular_detector_samplepos.png

...y/instr/det/rectangular-detector/xy_plane_with_labels.png

hugo/content/py/instr/det/resolution/index.md

hugo/content/py/instr/det/spherical-detector/index.md

...ntent/py/instr/det/spherical-detector/phi_alpha_plane.png

...instr/det/spherical-detector/phi_alpha_plane_variable.png

...nt/py/instr/det/spherical-detector/spherical_detector.png

hugo/content/py/instr/det/two_detectors.png

hugo/content/py/sim/gisas/_index.md

hugo/content/py/sim/offspec/index.md

hugo/content/py/sim/reflectometry/_index.md