Update the docs about Journald support.

[belimawr]

This PR updates the docs stating in which Container variantes the Journald dependencies are shipped, allowing Elastic-Agent to run inputs/integrations that read from journald like: system, journald and iptables.

The changes on this PR need to be released together with elastic/elastic-agent#7995 and its backports.

The docs can be previewed at: https://docs-v3-preview.elastic.dev/elastic/docs-content/pull/1272/reference/fleet/install-elastic-agents#flavors-for-container-package-installs

[belimawr]

@kilfoyle this PR introduce docs that are only valid for the next release, we're adding journalctl to our docker images. What is the best what to indicate/group the docs by version?

[kilfoyle]

this PR introduce docs that are only valid for the next release, we're adding journalctl to our docker images. What is the best what to indicate/group the docs by version?

Good question! The goal is to have different versions included in the same set of docs. When we add a new page or section that applies to a specific version, we can use these applies_to tags to indicate that. For changes that don't include a new section or page, we're supposed to mention the version right in the text, so I added Beginning with version 9.1, in your change above.

To avoid including future (say 9.1) changes in the current docs, the plan is to introduce next and current branches. That's not ready yet, however, so for now I'd say please don't merge 9.1 changes until the release goes live. I know this is inconvenient, but we will find a better approach, whether through branching, feature flags, or something else.

---

Add: I checked with another team and, for now, they're also staging their docs PRs affecting 9.1, with a plan to merge them all when the release goes live.

[github-actions]

🔍 Preview links for changed docs:

• reference/fleet/install-elastic-agents.md

🔔 The preview site may take up to 3 minutes to finish building. These links will become live once it completes.

[Copilot]

Pull Request Overview

This PR updates the documentation to clarify which containerized Elastic Agent flavors include journald dependencies, and adds the appropriate version annotation.

• Adds a {applies_to} directive to note that journald support in the servers flavor is available since GA 9.1.
• Moves the journald dependency note out of the complete flavor paragraph and into its own line.
• Separates concerns of synthetics dependencies and journald support for clarity.

Comments suppressed due to low confidence (1)

reference/fleet/install-elastic-agents.md:118

• [nitpick] Consider adding a {applies_to} directive here to indicate that journald support in the complete flavor is also available starting from GA 9.1, for consistency with the servers flavor.

The complete {{agent}} flavor also includes the [journald](https://www.freedesktop.org/software/systemd/man/latest/systemd-journald.service.html) dependences necessary to use the [journald input](beats://reference/filebeat/filebeat-input-journald.md).

[karenzone]

LGTM! 🚀

[karenzone]

@belimawr, based on our versioning discussions, it sounds like we should add a 9.1 label so that we can revisit if necessary when we nail down our versioning strategy.

[belimawr]

@belimawr, based on our versioning discussions, it sounds like we should add a 9.1 label so that we can revisit if necessary when we nail down our versioning strategy.

What do you mean? How is this label added? Isn't the applies to enough?

{applies_to}`stack: ga 9.1`

[github-actions]

🔍 Preview links for changed docs

• reference/fleet/install-elastic-agents.md