actually add docs page

docs/src/man/leastsquares.md

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+# Least squares problems
+
+Least square problems take the form of finding `x` that minimises `norm(b - A*x)` where
+`A` should be a linear map. As opposed to linear systems, the input and output of the linear
+map do not need to be the same, so that `x` (input) and `b` (output) can live in different
+vector spaces. Such problems can be solved using the function `lssolve`:
+
+```@docs
+lssolve
+```