Add docs about the Hosts metrics tab

[dedemorton]

Closes #3788.

Also adds a couple other additional (minor) changes:

• Cleans up some outdated text from other subtle UI changes.
• Moves the screenshots about host details tabs to appear after the text that explains the tab. Why? Users looking at the UI probably don't need the screenshot and would prefer to see text. Other users (who are maybe trying to figure out what is possible) will still see the screenshot but it won't interfere with the users ability to scan and find info.
• Cropped out the search bar in some of the screen captures. Why? Having inconsistent timestamps at the top of each screenshot looked weird plus didn't add a lot since the search field is not unique to each screen. Note that I didn't retake all the screenshots...just cropped them.

TODO (after merging):

• Port to serverless. Add docs about the Hosts metrics tab to serverless #3955

[github-actions]

A documentation preview will be available soon.

• 🔨 Buildkite builds
• 📚 HTML diff
• 📙 Preview page

Request a new doc build by commenting

• Rebuild this PR: run docs-build
• Rebuild this PR and all Elastic docs: run docs-build rebuild

_{run docs-build is much faster than run docs-build rebuild. A rebuild should only be needed in rare situations.}

_{If your PR continues to fail for an unknown reason, the doc build pipeline may be broken. Elastic employees can check the pipeline status here.}

[mergify]

This pull request does not have a backport label. Could you fix it @dedemorton? 🙏
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.

[dedemorton]

This PR is ready for review now!