Add variables. in documentation links

docs/src/defining_sequence.md

docs/src/implemented_sequences.md

docs/src/scanners.md

src/components/abstract_types.jl

src/components/instant_gradients.jl

src/components/pulses/constant_pulses.jl

src/components/pulses/instant_pulses.jl

src/components/pulses/sinc_pulses.jl

src/containers/abstract.jl

src/containers/base_sequences.jl

src/containers/building_blocks.jl

src/parts/epi_readouts.jl

src/parts/helper_functions.jl

src/parts/trapezoids.jl

src/pathways.jl

src/sequences/diffusion_spin_echoes.jl

src/sequences/gradient_echoes.jl

src/sequences/spin_echoes.jl

src/variables.jl