docs: describe the published container tags accurately

[Eljees]

Fixes #2398

The getting-started guide says the latest tag tracks the development branch, but that tag does not
exist on the registry — which is why podman pull quay.io/projectquay/clair:latest returns
manifest unknown.

Current tags on quay.io/projectquay/clair (via https://quay.io/api/v1/repository/projectquay/clair/tag/,
fetched today):

nightly              Tue, 28 Jul 2026 06:21:50 -0000
nightly-2026-07-28   Tue, 28 Jul 2026 06:21:53 -0000
nightly-2026-07-25   ...
4.9.0                Wed, 10 Dec 2025 18:38:54 -0000
4.8.0                Wed, 09 Oct 2024 20:22:40 -0000
4.7.4 ... 4.4.4      (release tags)

So the development branch is published as nightly, with dated nightly-YYYY-MM-DD tags kept alongside,
and releases as version tags. The doc now says that, and states explicitly that there is no latest tag,
so the next person doesn't have to find out via manifest unknown.

Documentation-only change; no code touched.

AI-assisted (LLM used for drafting); the tag listing above was fetched and checked by me.

[Eljees]

Ping - open since 28 July. Docs-only: the container tags described in the docs do not match what is actually published. Happy to correct the patch if I have misread the release process.

[Eljees]

I re-checked this today and the mismatch is still there, so the patch is still needed.

Documentation/howto/getting_started.md on main still reads "The latest tag tracks the git development branch". The tags actually published on quay.io/projectquay/clair right now are nightly (pushed this morning), the dated nightly-YYYY-MM-DD series, and the release tags 4.9.0, 4.8.0, 4.7.4/4.7.3/4.7.2/4.7.1/4.7.0, 4.6.1, 4.6.0. There is no latest; the only tag containing that word is quaybackstop-latest, which is a different image line.

The file has had no commits since this PR was opened, so the change still applies unchanged. It is three lines of documentation, and I am happy to reword it or rebase if you would rather phrase it differently.