Doc.: Link references to grammar productions to declarations of productions

[dsbos]

The grammar documentation at https://docs.scala-lang.org/scala3/reference/syntax.html would be a lot easier to traverse if the references to productions (i.e., names of non-terminals) were rendered as links to (the rendering of) the declarations of those productions.

(Ideally, each declaration would also provide some way to navigate to the (sometimes) multiple references to that declaration. Perhaps the tail end of the document could be an index by production name, with each production's entry listing links back to the references to the production, and with the production's declaration rendered as a link to the productions index entry. Then a reader could easily see both 1) the details of things used by a given production, and 2) where a given production is used elsewhere in the grammar.)