Improve docs for guides and getting started #5039

Job	Run time
clang-format	21s
copyright_headers	17s
cppcheck	3m 49s
forbidden_types	19s
rstcheck	24s
unused_names	50s
vale	36s
black	27s
flake8	21s
isort	10s
lychee_IGNORED	28s
pydocstyle	19s
mypy	38s
pylint	35s
sphinx-rtd	3m 22s
sphinx-conda	0s
build_macos (macos-latest, clang, openmp, mpi, python, gsl, ltdl, boost, hdf5, optimize, warning)	18m 23s
build_linux (ubuntu-20.04, gcc, boost, optimize, warning)	11m 52s
build_linux (ubuntu-20.04, gcc, openmp, python, gsl, ltdl, boost, optimize, warning)	18m 7s
build_linux (ubuntu-20.04, gcc, mpi, python, gsl, ltdl, boost, optimize, warning)	19m 7s
build_linux (ubuntu-20.04, gcc, openmp, mpi, python, gsl, ltdl, boost, hdf5, sionlib, libneurosim...	26m 37s
	1h 47m 2s

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