Clarify meaning of \odot in docs

[siboles]

Just a minor comment but I struggled to understand the meaning of the \odot operator you have defined. It appears to behave like a double contraction (at least from its use in the weak form definitions). Or is it specific to contraction of symmetric tensors? It may be helpful to explain its meaning somewhere in the introduction, perhaps in Einsteinian notation.

[zdq3157870]

I found the definition of \odot in Gridap.jl/src/TensorValues/Operations.jl. It's the same as the inner product.

[mSamiolo]

The definition of \odot is the following:

Inner product between tensors (summed pairwise product of all elements)