Writing words about handling multivariate data (dfm#30)

* starting to write words about multivariate data

* [pre-commit.ci] auto fixes from pre-commit.com hooks

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* more words in multivariate tutorial

* [pre-commit.ci] auto fixes from pre-commit.com hooks

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* fixing test for linear transform

* adding troubleshooting document

* fixing docs syntax error

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

tiny docs updates

tiny docs updates

fixing dtype issues