Add troubleshooting info for 'not a known icon'

What does this MR do?

In gitlab-docs#1777 (closed), I'm updating the html-lint Docker image. The copy of the SVG repository in the existing Docker image did not have the SVG that a frontend engineer needed, so !147384 (merged) threw pipeline errors. She tried to self-solve the problem, but the only explanation of the problem and its solution is in https://gitlab.com/gitlab-org/gitlab-docs/-/blob/main/doc/troubleshooting.md — not a place she would have known to look.

As part of the cleanup, let's iterate to make this process better. Let's add enough troubleshooting information about this error into the main gitlab repo to point users to the right place. Make any changes you see fit; I have no strong attachment to any of the text.

Separately, I'll also propose an improvement to the error message shown in the pipeline itself.

Related issues

• Related to Frontend: Add badge for components (!147384 - merged) where the engineer was confused by pipeline failures
• Related to Update the html-lint Docker image (gitlab-docs#1777 - closed) which tracks the work of updating the images

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