Add index.html stub to try out gh-pages #436

Job	Run time
build (g++-10, Debug, 17)	39s
build (g++-10, Release, 17)	44s
build (clang++-10, Debug, 17)	53s
build (clang++-10, Release, 17)	56s
build (g++-7, Debug, 17, sudo apt-get install g++-7)	46s
build (g++-11, Debug, 20, sudo apt-get install g++-11)	1m 2s
build (clang++-6.0, Debug, 17, sudo apt-get install clang-6.0)	1m 3s
build (clang++-7, Debug, 17, -stdlib=libc++, sudo apt-get install clang-7 libc++-7-dev libc++abi-...	53s
build (clang++-7, Release, 17, sudo apt-get install clang-7)	55s
build (clang++-11, Debug, 20, -stdlib=libc++, sudo apt-get install clang-11 libc++-11-dev libc++a...	1m 4s
build (clang++-11, Release, 20, sudo apt-get install clang-11)	1m 6s
build (sanitizers, clang++-12, Debug, 20, -O1 -g -fsanitize=address,undefined,leak,integer -fno-o...	1m 35s
build (valgrind-memcheck, g++-10, Debug, 17, -O1 -g -fno-omit-frame-pointer -fno-optimize-sibling...	53s
build (coverage, g++-10, Debug, 17, -g -O0 --coverage -fno-inline, sudo apt-get install -y lcov, ...	45s
	13m 14s

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