Some documentation fixes

Christoph Lehmannrequested to merge
chleh/ogs:fix-constitutive-relation-doc into master

This MR fixes several small issues with documentation:

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Added <constitutive_relation> docu to THM, TRM, TH2M

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Restructured non-linear solver docu a bit

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TODO

We'll need some further improvements in the future, e.g.:

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Edited by Christoph Lehmann

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