[DOCS] Document the new UI for installing and upgrading prebuilt detection rules

[banderror]

Epic: https://github.com/elastic/security-team/issues/1974 (internal)
New workflow implementation PR: elastic/kibana#158450

Summary

In 8.9.0, we have replaced our previous prebuilt rule installation and upgrade workflow.

• Previously, it was only possible to upgrade all installed prebuilt rules and install all new prebuilt rules at once. It was not possible to see what a new detection rules package version contains.
• Now, users can:
  • See what new rules they can install (at this point, we show only the rule name and a few other fields in the table).
  • Choose which rules to install. They can install them all at once (as before), one by one, or bulk install selected rules.
  • See what installed rules can be upgraded.
  • Choose which rules to upgrade. They can upgrade them all at once (as before), one by one, or bulk upgrade selected rules.

On the Rule Management page, there's now a new table that shows rules that can be upgraded. Also, we added a separate page from which users can install new prebuilt rules.

Videos

The new rule installation workflow:

New.Rule.Installation.Workflow.mp4

The new rule upgrade workflow:

New.Role.Upgrade.Workflow.mp4

[joepeeples]

@banderror @xcrzx We previously opened #3147 for this, which @banderror pointed out refers to features not yet available (customizing prebuilt rules). We'll save that issue for later, and use this issue for the work going into 8.9. Also, I added a few labels for docs team tracking.

[joepeeples]

This affects a couple of screenshots on Manage detection rules and Monitor and troubleshoot rule executions (maybe others?). Originally, I was going to update these images as part of this PR, but @nastasha-solomon and I discussed and they'll be part of #3486 instead.