ASKurz/Statistical_Rethinking_with_brms_ggplot2_and_the_tidyverse: Expand the multinomial and negative binomial sections

Noteworthy changes include: substantial expansions to sections 10.3.1 (multinomial regression) and 11.3.2 (negative binomial regression), the addition of a new section in Chapter 15 (15.9) encouraging others to code in public, refitting all models with the current official version of brms, version 2.12.0, discussions (8.3.2) on the new Bulk_ESS and Tail_ESS summaries of HMC effective sample size (Vehtari et al., 2019), saving all fits as external files in the new GitHub fits folder, primarily with the file argument, extensive use of the patchwork package for combining ggplots, improving/updating some of the tidyverse code (e.g., using tidyr::crossing()), updates to the brms::custom_family()-related code in 11.3.1 to match brms 2.11.0 updates, replacing the depreciated brms::marginal_effects() with brms::conditional_effects() (see issue #735), replacing the depreciated brms::stanplot() with brms::mcmc_plot(), increased the plot resolution with fig.retina = 2.5, refreshed hyperlink