Merge branch 'web/16470-better-install-vs-upgrade' into web/16398-3.13-release-notes (Will-fix: #16470)

It triggers the presence of some <p> tags inside <li> for the first lines…

This commit, along with a21d0e2f, fix an issue
while following the release process documentation; apologies for the troubles.

d34983f6 made the process too error prone: it's too easy to follow "If preparing
anything but a final release" while one should follow "Else, if preparing
a final release", and it just happened. Let's de-duplicate this and
do the right thing depending on the value of $DIST.

It's probably less dangerous and confusing to show upgrade first to
people who want to install than to show install first to people who want
to upgrade as they could erase their persistence is their follow the
installation instructions

- Separate them more
- Clarify the impact on persistent storage
- Put on the same level as download only in release notes

Superseded by sprint.

The current schedule has both 3.14 and 3.15 as bugfix releases.