Add LU solver documentation #6197

Job	Run time
build-cpp-test-macos (debug)	8m 32s
Build sdist and set version	20s
build-cpp-test-macos (release)	9m 1s
Build and test in Conda (ubuntu)	2m 37s
build-cpp-test-linux (debug, gcc)	11m 46s
Build and test in Conda (windows)	7m 3s
build-cpp-test-linux (debug, clang)	8m 38s
build-cpp-test-windows (debug, msvc)	11m 2s
build-cpp-test-linux (release, gcc)	11m 13s
build-cpp-test-windows (debug, clang-cl)	12m 41s
build-cpp-test-linux (release, clang)	11m 46s
build-cpp-test-windows (release, msvc)	15m 6s
build-cpp-test-windows (release, clang-cl)	14m 13s
build-and-test-python (linux-x64)	8m 26s
build-and-test-python (linux-aarch64)	4m 5s
build-and-test-python (macos)	4m 51s
build-and-test-python (windows)	7m 45s
publish-wheels	6s
	2h 29m 11s

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