Sphinx doc missing dependencies

[SimonRit]

When

• compiling sphinx doc make sphinx_doc
• creating a new README.md in an application subdirectory (e.g., rtkfourdconjugategradient)
• adding a line in the toctree in rtk/applications/README.md
• compiling sphinx doc make sphinx_doc (without cleaning first)
• checking the result in sphinx/_build/index.html

The TOC is ok for the main page sphinx/_build/applications/README.html but it is not updated in application HTML pages (e.g., sphinx/_build/applications/rtkfdk/README.html).

[SimonRit]

cc @cyrilmory