Adding Installation instruction into PecanPy documentation Website

[SakaizaRajery]

Hello,

I wanted to note that having the installation instructions both on Github and on the PecanPy documentation website (https://pecanpy.readthedocs.io/en/latest/pecanpy.html) would be useful.

Thank you,

Sakaiza

[RemyLau]

Hi @SakaizaRajery thank you for your suggestion! I will add the installation instruction to the documentation page soon!