Update links in the picker to the new website

[mririgoyen]

We need to update the extension to start pointing to the new site. Here's all the places that I have found that should be updated (I might have missed some!).

• The Header's link to go to https://pictogrammers.com/library/mdi
• Each icon's "View on MDI" should now link to https://pictogrammers.com/library/mdi/icon/{ICON_NAME}
• "Copy markdown preview"s content should be updated to...

  [![ICON_NAME](https://pictogrammers.com/library/mdi/icon/{ICON_NAME)](https://pictogrammers.com/library/mdi/icon/{ICON_NAME)

We should probably also close https://github.com/Pictogrammers/Browser-Icon-Picker/issues/8 as wontfix.

[quentin-st]

+1, thanks for the issue!

[quentin-st]

Done, thanks!