Document `print_tree` for `RuleNode`s

[ReubenJ]

Somewhere in the documentation, we should highlight that you can pretty-print RuleNodes in tree form using print_tree from AbstractTrees. Assuming Herb-AI/HerbCore.jl#39 is merged, this will be available from within Herb without the need for an extra using AbstractTree.

Also, while not specific to Herb, it'd be nice to note that Exprs can be pretty-printed in the same way.

julia> using Herb
julia> using AbstractTrees: print_tree # until HerbCore.jl#39 is merged

julia> g = @csgrammar begin
                   Int = 1 | 2
                   Int = 3 + Int
               end

julia> rn = @rulenode 3{3{3{2}}}

julia> print_tree(rn)
3
└─ 3
   └─ 3
      └─ 2

julia> print_tree(rulenode2expr(rn, g))
:(3 + (3 + (3 + 2)))
├─ :+
├─ 3
└─ :(3 + (3 + 2))
   ├─ :+
   ├─ 3
   └─ :(3 + 2)
      ├─ :+
      ├─ 3
      └─ 2