Fix build system pages for devs

Santiago Ospina De Los Ríosrequested to merge
feature/fix-build-system-for-devs into master

The build system documentations for developers is quite broken.

  • The list of sphinx does not appear although is being mentioned explicitly.
  • The link to the dune book is more for users and not for developers.
  • The link to de build system documentation for developers in the user documentation is broken.
  • The released documentations is too old (2.7)

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