Updated and improved README file

Carlos Mastallirequested to merge
cmastall/crocoddyl:topics/readme into devel

These are the tasks in this PR:

  1. Added Numpy and example-robot-data as dependencies.
  2. Included links for all dependencies.
  3. Fixed a rendering problem with the code for running talos_arm example.
  4. Added links to all Jupyter notebooks.

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