Replace todo.md by real page

[mathbagu]

• Do you want to request a feature or report a bug?
  Bug

• What is the current behavior?
  If a user follow a todo link, the sidebar is closed because the todo.md page has no category.

• If the current behavior is a bug, please provide the steps to reproduce and if possible a minimal demo of the problem

• What is the expected behavior?
  If we create an empty page, using the todo layout (like for loadflow, the menu is open and the corresponding item is selected

• What is the motivation / use case for changing the behavior?
  Make the navigation easier

• Please tell us about your environment:

  • PowSyBl Version: ...
  • OS Version: ...

• Other information (e.g. detailed explanation, stacktraces, related issues, suggestions how to fix, links for us to have context, eg. stackoverflow, spectrum, etc)

(if a question doesn't apply, you can delete it)

[mathbagu]

Not relevant since the sidebar doesn't exist anymore