I'll need that to be able to move the boot instructions for PC out of a
toggle and always displayed: for them to be hidden for Mac and visible
for PC, they have to be entirely inside an HTML tag. Until now it was
not needed only because we used a toogle who switch was itself only
displayed for PC.

Having dedicated inlines should make the code cleaner.

Closes: #16679

This was introduced in commit:3329d12c.

Reviewing guidelines: document how to check APT overlay suites without direct access to our reprepro instance.

We now have reviewers without commit/upload rights.

And while I'm at it, replace obsolete mention of "the APT suite" with up-to-date
instructions: branches now need to explicitly enable APT overlays.