Split installation guide into 3 sections: env, deps and troubleshooting

[ureyes84]

Currently we have one h1 header "Installation" and multiple h2 sub headers.

It would be good to have three h2 headers:

• setting up virtual env, with 2 sub sections: virtualev vs conda
• installing the tool, with 2 subsections: pip vs setup.py
• Troubleshooting

[joristaglio]

Done!