More development docs related to interpolations

[termi-official]

I added more information about the interpolations that I could reconstruct over the last months + some initial grid docs related to dof management and interpolations. Note that the AbstractCell interface is not really well-specified at this point, so I just used the assumptions taken on Cell to document it for now.

[codecov-commenter]

Codecov Report

Patch coverage: 27.27% and project coverage change: -0.09 ⚠️

Comparison is base (84e9aef) 93.05% compared to head (f1ecb26) 92.97%.

❗ Current head f1ecb26 differs from pull request most recent head f183adb. Consider uploading reports for the commit f183adb to get more accurate results

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #611      +/-   ##
==========================================
- Coverage   93.05%   92.97%   -0.09%     
==========================================
  Files          28       28              
  Lines        4277     4270       -7     
==========================================
- Hits         3980     3970      -10     
- Misses        297      300       +3     

Impacted Files	Coverage Δ
src/Grid/grid.jl	90.88% <ø> (ø)
src/interpolations.jl	90.47% <20.00%> (-1.01%)	⬇️
src/Dofs/DofHandler.jl	90.45% <100.00%> (ø)

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