feat!: render_docs() renders to gfm output

[etiennebacher]

Fixes #331. Apart from extra spaces, it seems that this affects equations and symbols.

In roxygen2 syntax:

#' Some code with weird symbols: `pl$when(condition)` and `pl$then(output)`
#'
#' Some equations: \eqn{\partial Y / \partial X = a + \varepsilon/2}

In Rd file:

Some code with weird symbols: \code{pl$when(condition)} and \code{pl$then(output)}

Some equations: \eqn{\partial Y / \partial X = a + \varepsilon/2}

render_docs() before:

"Some code with weird symbols: <code>pl$when(condition)</code> and"              
"<code>pl$then(output)</code>"  

"Some equations: ∂*Y*/∂*X* = *a* + *ε*/2"       

render_docs() after:

"Some code with weird symbols: <code>pl\\$when(condition)</code> and"            
"<code>pl\\$then(output)</code>"        

"Some equations: $\\partial Y / \\partial X = a + \\varepsilon/2$"