[DOCS-15218] Incorporate feedback on Configure Custom Logs page

[evazorro]

What does this PR do? What is the motivation?

Fixes DOCS-15218

Incorporates some post-merge feedback on the new Configure Custom Logs topic from #38049.

• Added some extra context to the intro
• Added links in a few places to how to upgrade your Agents, and a link to learn more about what the different log collection fields do
• In the warning that containerized workflows aren't supported, added a link to docs on Kubernetes log collection
• Added a stub section for verifying that log collection config worked
• (small) Removed a couple stray </a> tags that I noticed

Merge instructions

Merge readiness:

• Ready for merge

Additional notes

Open questions (cc @ethandebnath7702):

• Does the Agent need a restart after Fleet Automation pushes log configuration, the way the manual conf.d flow does?
• Does a pushed configuration take effect on an Agent that still has logs_enabled: false? If not, log collection belongs in Prerequisites.

[evazorro]

@ethandebnath7702 do we want to add verification steps, or just link to the troubleshooting guide? Can you assign someone on dev to provide those steps?

[github-actions]

Preview links (active after the build_preview check completes)

Modified Files

• https://docs-staging.datadoghq.com/eva/DOCS-15218-configure-logs-feedback/agent/fleet_automation/configure_agents
• https://docs-staging.datadoghq.com/eva/DOCS-15218-configure-logs-feedback/agent/fleet_automation/configure_integrations
• https://docs-staging.datadoghq.com/eva/DOCS-15218-configure-logs-feedback/agent/fleet_automation/configure_logs
• https://docs-staging.datadoghq.com/eva/DOCS-15218-configure-logs-feedback/agent/logs/