Add the Extract the log level field section

[mdbirnstiehl]

This PR closes Issue 3121 and adds the Extract the log.level field to the Stream a log file documentation.

[github-actions]

A documentation preview will be available soon:

• 📚 HTML diff
• 📙 Observability Guide

[mergify]

This pull request does not have a backport label. Could you fix it @mdbirnstiehl? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-/d./d is the label to automatically backport to the /d./d branch. /d is the digit
  NOTE: backport-skip has been added to this pull request.

[ruflin]

Follow up request (out of scope of this PR): Show the user how to do a terms agg on the log.level field to see how many docs there are for a certain time period.

[ruflin]

Overall direction LGTM, left one comment around mappings which looks off. As usual, lets get this in quickly and then iterate on it.

[mergify]

This pull request is now in conflict. Could you fix it @mdbirnstiehl? 🙏
To fixup this pull request, you can check out it locally. See documentation: https://help.github.com/articles/checking-out-pull-requests-locally/

git fetch upstream
git checkout -b extract-log-level upstream/extract-log-level
git merge upstream/main
git push upstream extract-log-level

[bmorelli25]

🚢