Add documentation

[romainljsimon]

This is the first commit to start the documentation process. The documentation will not build and deploy because ParticlesMC is not on the official registry...yet.
However one can build the documentation locally by running:

julia -project=docs/ docs/make.jl 

after having developed ParticlesMC:

] dev https://github.com/TheDisorderedOrganization/ParticlesMC.git

Closes #12

[codecov]

Codecov Report

❌ Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/utils.jl	0.00%	2 Missing ⚠️

Files with missing lines	Coverage Δ
src/ParticlesMC.jl	10.11% <ø> (ø)
src/atoms.jl	77.19% <ø> (ø)
src/molecules.jl	71.18% <ø> (ø)
src/moves.jl	54.95% <ø> (ø)
src/utils.jl	86.20% <0.00%> (-6.39%)	⬇️

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