write documentation in rst

[12rambau]

We have build documentation for the SEPAL platform and BFAST explorer is one of the (many) application that we provide to our users.

The documentation is buid using sphinx so I needed .rst file to parse it with the other. That would have been to complicated to parse it again so I wrote a separated .rst.

• Using "Bfast explorer" as a main title
• using substitution rules for the inlin icons
• using note admonition to replace the htm text
• using thumbnail to display images (as the doc is created elswhere I used absolute path to image instead of the .md relative path)
  doc.sepal.io is currently point ing to my fork of your repository but I woudl be happy to point to yours if you accept my PR !

https://docs.sepal.io/en/latest/modules/dwn/bfast_explorer.html