[docs] Fix image paths for docs-assembler

[colleenmcginnis]

Fixes image paths to work with docs-assembler.

Notes for the reviewer:

• I was not able to get images in reference, extend, or release-notes to work using the :::{image} syntax because it seems to resolve differently than the Markdown ![]() syntax. We should address this in docs-builder, but in order to get images working as soon as possible, I've used Markdown syntax and left us a TO DO in a code comment to add back the screenshot class where applicable.
• Can you please add the appropriate labels needed for backporting?

[mergify]

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

• backport-8./d is the label to automatically backport to the 8./d branch. /d is the digit.
• backport-8.x is the label to automatically backport to the 8.x branch.
• If no backport is necessary, please add the backport-skip label

[github-actions]

📃 DOCS PREVIEW ✨ https://logstash_bk_17566.docs-preview.app.elstc.co/diff

[karenzone]

LGTM

[github-actions]

@Mergifyio backport 9.0

[mergify]

backport 9.0

✅ Backports have been created

• #17568 [9.0] (backport #17566) [docs] Fix image paths for docs-assembler has been created for branch 9.0