[r21_linkcheck] Fix broken links in r21 webdocs (#699) ()

hugo/content/ex/fit/bayesian.md

hugo/content/ex/fit/external-minimizer-with-plotting.md

hugo/content/ex/fit/fit-with-uncertainties.md

hugo/content/ex/fit/galaxi.md

hugo/content/ex/fit/gisas-fit2d.md

hugo/content/ex/fit/minimizer-settings.md

hugo/content/ex/fit/polarized-spinasymmetry-fit.md

hugo/content/ex/fit/reflectometry-honeycomb.md

hugo/content/ex/fit/reflectometry-pt-layer.md

hugo/content/ex/result/export/_index.md

...content/ex/result/export/axes-in-different-units/index.md

hugo/content/ex/result/export/more.md

hugo/content/ex/sample/roughness/specular_2models.md

hugo/content/ex/sample/roughness/specular_default.md

hugo/content/ex/sim/specular.md

hugo/content/installation/py/linux.md

hugo/content/ref/fit/minimizers.md

hugo/content/ref/instr/beam/_index.md

hugo/content/ref/instr/beam/angular-divergence/index.md

hugo/content/ref/instr/beam/footprint/index.md