Proposes a tutorial on how to implement a manifold

[kellertuer]

My aim is to maybe slowly extend this and write more tutorials in forthcoming PRs.

[codecov]

Codecov Report

Merging #190 into master will decrease coverage by 0.07%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #190      +/-   ##
==========================================
- Coverage   94.37%   94.30%   -0.08%     
==========================================
  Files          50       50              
  Lines        3201     3212      +11     
==========================================
+ Hits         3021     3029       +8     
- Misses        180      183       +3     

Impacted Files	Coverage Δ
src/manifolds/Sphere.jl	97.72% <ø> (+0.08%)	⬆️
src/manifolds/Symmetric.jl	96.82% <0.00%> (-1.59%)	⬇️
src/manifolds/Euclidean.jl	91.86% <0.00%> (-0.92%)	⬇️
src/groups/metric.jl	84.61% <0.00%> (-0.76%)	⬇️
src/manifolds/PowerManifold.jl	95.83% <0.00%> (-0.45%)	⬇️
src/Manifolds.jl	100.00% <0.00%> (ø)
src/manifolds/GeneralizedStiefel.jl	100.00% <0.00%> (ø)
src/manifolds/GeneralizedGrassmann.jl	100.00% <0.00%> (ø)
src/manifolds/Circle.jl	99.23% <0.00%> (+0.01%)	⬆️
... and 2 more

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[mateuszbaran]

This is a good idea 👍 . I left a few comments and I'll look at it again later.

[kellertuer]

Thanks, I'll merge this so it's on dev and with the next feature it'll be on the main docs then.