Add a header to bootstrap script with some infos

[lefou]

Since #6117, the bootstrap script don't have any header comment. This makes working with them inconvenient, esp. when maintaining older projects, and you want to know where the script is coming from or which version it is.

Script should contain the following info:

• Mill Project + URL
• A link to the download page
• A link to the online documentation of the script (essentially what was removed as part of #6117)
• A version