enahnce vignettes clarity

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vignettes/01-theoryBehindHtrfit.Rmd

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 # HTRfit simulation workflow
 
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 In this modeling framework, counts denoted as $K_{ij}$ for gene i and sample j are generated using a negative binomial distribution. The negative binomial distribution considers a fitted mean $\mu_{ij}$ and a gene-specific dispersion parameter $dispersion_i$.