Major updates to the jigdo and mirroring pages

* Update the jigdo homepage link
* jigdo is not necessarily the future of downloads, so don't pretend
* Mention CD/DVD rather than just CD
* Clarify better about jigdo-lite/jigdo-file
* Remove dead text about old unofficial images
* Only mention sha256 and sha512 checksums
* Massive updates to the mirroring section, it was very outdated

Closes: #945248