Provide instructions to Windows users about uninstalling previous Dangerzone versions

[apyrgio]

As a result of #929, Windows users who have install Dangerzone before will be greeted with a message that says they need to uninstall previous versions, in order to install the new one. In order to help these users, here's what we can do:

• In our website, keep the download button for the Windows MSI as is.

• If users have not installed Dangerzone before, they will not be affected.

• Write a descriptive error message, for the users who have Dangerzone installed. Something like:

  We detected that a previous Dangerzone version is installed in your system. Dangerzone versions prior to 0.9.0 must be uninstalled from "Programs and Features", before installing a new version. Read more about this change in:

• Add a section in our dangerzone.rocks website or our GitHub wiki page where we can explain with screenshots how to uninstall a previous Dangerzone version, and install the new one. We can also link to this PR for more context.

Hopefully this will be a smooth transition as any.

Originally posted by @apyrgio in #929 (comment)