Improve documentation (Installation)

It can be difficult to install insarviz for an user that is not geeky:

• how to install conda (and explain why we need it), add link to installation doc, explain the difference between conda and anaconda ?
• explain that a gitlab account is required to clone (as we provide the clone ssh version), or provide a clone command via HTTPS
• in "Install according to your usage" add the information that pip install . must be done in insarviz folder, and put "navigate to within the top-level insarviz folder" in bold
• pour windows specifier de lancer un prompt anaconda (anaconda powershell prompt) avant de faire les commandes conda