Move EllipticSurface to src and document it.

[simonbrandhorst]

Move EllipticSurface and BlowupMorphism to src.
Document EllipticSurface.

[simonbrandhorst]

@HechtiDerLachs @afkafkafk13
I had to move the blowups out of experimental.
Could you please add them to the docu in another PR?

[codecov]

Codecov Report

Attention: Patch coverage is 79.97947% with 390 lines in your changes missing coverage. Please review.

Project coverage is 84.50%. Comparing base (f2329a8) to head (6580c68).
Report is 9 commits behind head on master.

Files with missing lines	Patch %	Lines
...raicGeometry/Surfaces/EllipticSurface/Morphisms.jl	63.09%	217 Missing ⚠️
...ometry/Surfaces/EllipticSurface/EllipticSurface.jl	83.64%	121 Missing ⚠️
...cGeometry/Surfaces/EllipticSurface/NeighborStep.jl	91.92%	42 Missing ⚠️
...cGeometry/Schemes/CoveredProjectiveScheme/Types.jl	78.37%	8 Missing ⚠️
...Geometry/Surfaces/EllipticSurface/MoveMeToHecke.jl	94.11%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4294      +/-   ##
==========================================
+ Coverage   84.48%   84.50%   +0.02%     
==========================================
  Files         641      645       +4     
  Lines       85541    85538       -3     
==========================================
+ Hits        72266    72286      +20     
+ Misses      13275    13252      -23     

Files with missing lines	Coverage Δ
...cGeometry/Schemes/BlowupMorphism/BlowupMorphism.jl	84.15% <ø> (ø)
.../AlgebraicGeometry/Schemes/BlowupMorphism/Types.jl	92.59% <ø> (ø)
...CoveredProjectiveScheme/CoveredProjectiveScheme.jl	89.18% <ø> (ø)
...lgebraicGeometry/Surfaces/EllipticSurface/Types.jl	100.00% <100.00%> (ø)
src/deprecations.jl	0.00% <ø> (ø)
...Geometry/Surfaces/EllipticSurface/MoveMeToHecke.jl	94.11% <94.11%> (ø)
...cGeometry/Schemes/CoveredProjectiveScheme/Types.jl	78.37% <78.37%> (ø)
...cGeometry/Surfaces/EllipticSurface/NeighborStep.jl	91.92% <91.92%> (ø)
...ometry/Surfaces/EllipticSurface/EllipticSurface.jl	83.64% <83.64%> (ø)
...raicGeometry/Surfaces/EllipticSurface/Morphisms.jl	63.09% <63.09%> (ø)

... and 8 files with indirect coverage changes