grid manual update and new docs for mesh reading

[koehlerson]

No description provided.

[codecov-commenter]

Codecov Report

Base: 91.99% // Head: 92.00% // Increases project coverage by +0.01% 🎉

Coverage data is based on head (e136fec) compared to base (523f700).
Patch has no changes to coverable lines.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #504      +/-   ##
==========================================
+ Coverage   91.99%   92.00%   +0.01%     
==========================================
  Files          22       22              
  Lines        3673     3703      +30     
==========================================
+ Hits         3379     3407      +28     
- Misses        294      296       +2     

Impacted Files	Coverage Δ
src/Dofs/MixedDofHandler.jl	86.39% <0.00%> (-0.92%)	⬇️
src/Grid/coloring.jl	92.43% <0.00%> (+0.12%)	⬆️
src/Dofs/DofHandler.jl	89.16% <0.00%> (+0.15%)	⬆️
src/Dofs/ConstraintHandler.jl	93.27% <0.00%> (+0.28%)	⬆️

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[koehlerson]

should the added section be moved to manual?

[KnutAM]

should the added section be moved to manual?

I think that would be good!
Then, perhaps it makes sense to rename the header to "How to obtain a mesh" (or grid),
and have sub-headers: generate_grid, FerriteGmsh.jl, FerriteMeshParser.jl?
I'm not sure if this should come before or after the description of the Grid type though.
First-time users would primarily be interested in how to obtain meshes. But part of the descriptions rely on basic knowledge about the Grid-type

[KnutAM]

Looks good, just a few minor things that I saw.
Perhaps we should change the PR name since this turned into restructuring the grid documentation as well.