Add docs for --elk-algorithm and examples for each one

[Talleyrand-34]

I am trying to test other layout algorithms to test if they perform better some diagrams, when i try them i cannot get them to work, even knowing that some are for special graphs as it is referred in issue terrastruct/d2#1417

I tries to adapt some examples from graphviz but they didn't compile.

Can some examples be added in the docs with the associated algorithm?