Fix warnings in `dune build @doc`

[triangle-man]

For example:

> dune build @doc

File "../../lib/.Whatwhat.objs/byte/whatwhat.odoc":
Warning: Failed to lookup type unresolvedroot(Stdlib__Map).Make(unresolvedroot(Stdlib).Int).t Parent_module: Unresolved apply

[yongrenjie]

This is an upstream issue which has to do with dune not being able to access other packages installed in the current switch, meaning that it can't access or cross-reference the docs of any dependencies (not even stdlib): https://github.com/ocaml/odoc/issues/794

The workaround I found (discussed here) was to not use dune build @doc but rather odig. On the first time, do:

opam install odig

# install the package into the current opam switch
cd whatwhat
dune build
dune build @install
dune install

# generate and view documentation
odig doc whatwhat

To subsequently update the docs:

dune build
dune install
odig doc -u whatwhat   # (update)

That runs without errors (if you use odig doc -v whatwhat it spews a load of warnings from the dependencies, but none from whatwhat itself).

I'll add this to the readme or wiki. Added to readme (in d7a777e) and wiki