fix(ci): the doc gate could not see a page it had just created

[Denis-hamon]

The gate could not see a page it had just created

make doc runs docgen, which writes one markdown page per cobra command. The
check that follows was git diff --exit-code, and git diff reports tracked
files only. A brand new command therefore produced a brand new file, which is
untracked, which the check cannot see.

That is not a hypothetical:

$ git ls-tree origin/main doc/ovhcloud_logout.md
$                                   # 935 pages tracked, this one absent

$ make doc && git diff --exit-code ; echo "rc=$?"
rc=0                                # green

$ git status --porcelain
?? doc/ovhcloud_logout.md           # what the gate did not see

logout shipped in #228 and its page has never been in the repository. The
gate was green on the pull request that added it, and has been green on every
pull request since.

The fix

git status --porcelain reports untracked files too, which is the whole point
of the check. The failure now prints what actually changed, so a drift in a
file that is not documentation does not read as a stale doc.

Verified by sabotage — reproducing exactly the state main is in today:

tree state	old gate	new gate
page generated, tracked	green	green
page generated, untracked	green	red

The step was left unscoped rather than narrowed to doc/. Scoping reads
cleaner, but git diff --exit-code also happened to catch a go.mod/go.sum
drift left by the go get ./... step above it, and narrowing the check would
have dropped that with nothing else covering it. Measured: go get ./...
leaves the tree clean today, so the broader check costs nothing.

Also in this commit

• doc/ovhcloud_logout.md, the page that was missing.
• Its entry in doc/ovhcloud.md, next to login.

One thing this does not fix, reported rather than decided

make doc ends with git checkout doc/ovhcloud.md, so the generated index is
discarded and the written one is maintained by hand. Nothing keeps it in sync,
and four commands are missing from it:

ovhcloud_browser.md
ovhcloud_completion.md
ovhcloud_logout.md      <- added here
ovhcloud_upgrade.md

A check that every top-level page is referenced in the index would catch this,
and would have caught logout twice over. It is not in this pull request
because whether completion belongs in a hand-curated index is an editorial
call for the maintainers, not a mechanical one.