Uniform treatment of sidebar

[panglesd]

This PR is a change in how we handle the sidebar.

The current status is that we give the sidebar the modules and the pages separately. This is good because it allows them to be rendered separately which can be readable, but this is bad because:

• It gives less flexibility in the sidebar/hierarchy (for instance in a monorepo we might want to hierarchize the modules) while letting it be quite uniform in the canonical case.
• The sidebar and the breadcrumbs do not correspond
• There are conflicts whenever the library roots are included in the pages roots
• This does not handle implementations!

This PR changes the sidebar so that it is generated from the hierarchy of both modules, pages, sources, ...
The good things are above. The bad things are:

• It is a bit harder to make it look good.
• The children_order is more important (to have the libraries at the end).

Here is a preview for such a sidebar for ppxlib.

(It seems to me that we will need to add the @toc-expand tags)

I still need to clean up the history but the code should be ok already, and the discussion can start!

[jonludlam]

Thanks for this! It breaks sherlodoc though - do you have a branch of that that's compatible?