add content discussing the ranges of pdfs #139

Forwarded from Morrison-Lab/rme#771, whose content moved here in Morrison-Lab/rme#1234 (d4ea51c).

The distribution content now lives in distributions.qmd (_subfiles/_sec-distributions.qmd) and the hazard function in random-variables.qmd (_subfiles/_def-hazard.qmd). Neither discusses the range of a pdf (for example, the Gaussian pdf's maximum $1/(\sigma\sqrt{2\pi})$ grows without bound as $\sigma \to 0$) or of a hazard function. rme's survival chapter also keeps its own hazard section, so the hazard part stays open in rme too.

Original issue

range of normal pdf (as a function of sigma)

range of hazard function

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