Use shared attributes for icons

[colleenmcginnis]

Description

⚠️ Do not merge until elastic/docs#3108 is merged ⚠️

Use shared attributes for icons instead of using images stored in each content repo.

Context: When we migrated serverless docs to AsciiDoc, we had to add SVGs for icons that were previously handled by the DocIcon component. To allow us to move more quickly, I added the SVGs to each content repo, but planned to centralize those icons in the /docs repo later (i.e. now 😄).

Documentation sets edited in this PR

Check all that apply.

• Stateful (docs/en/observability/*)
• Serverless (docs/en/serverless/*)
• Integrations Developer Guide (docs/en/integrations/*)
• None of the above

Related issue

elastic/docs#3108

Checklist

• Product/Engineering Review
• Writer Review

Follow-up tasks

Select one.

• This PR does not need to be ported to another doc set because:
  • The concepts in this PR only apply to one doc set (serverless or stateful)
  • The PR contains edits to both doc sets (serverless and stateful)
• This PR needs to be ported to another doc set:
  • Port to stateful docs: <link to PR or tracking issue>
  • Port to serverless docs: <link to PR or tracking issue>

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[mergify]

This pull request is now in conflict. Could you fix it @colleenmcginnis? 🙏
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 centralize-icons upstream/centralize-icons
git merge upstream/main
git push upstream centralize-icons