DOC Updates to SVM User Guide (scikit-learn#16769)

* WIP

* WIP

* Updates to the math section

* so apparently crammer-singer isn't consistent (http://www.cs.columbia.edu/\~rocco/papers/icml13.html)

* reorganized example links and described pictures

* Added references

* more refs

* Added scaler to docstring examples

* doc about dual coefs

* WIP

* note about shrinking and gram matrix

* ellipsis

* ellipsis again

* maybe fixed doc issue

* details for regression

* training error -> margin error

* Apply suggestions from code review

Co-Authored-By: Adrin Jalali <[email protected]>

* put back links + addressed rest of comments

* small detail about margin boundaries and SVs

* Addressed comments from Thomas and Adrin

Co-authored-by: Adrin Jalali <[email protected]>

Author: NicolasHug