Elaborate on link tag id prop for vscode-icon

[erikmueller]

I was banging my head against the wall for a couple of hours yesterday as I couldn't make icons to show up in my extension. My initial thought was an error in the way assets are bundled and resolved or some misconfigured CSPs but it was much simpler. Had i just copied the line from the icon documentation it probably would have worked out of the box.

Long story short, I was missing the id on the link tag. So after reading the implementation I understood what it's used for and why it's needed. I also found that there were at least 2 occurrences of the same issue (vscode-elements/elements#15 and vscode-elements/elements#125).

Let me know if you think stating the necessity a bit more explicitly and elaborating in the docs makes sense (it certainly would have helped me not overlooking this).
I'm also open for addressing this in a different way (like an Infobox or similar).

[bendera]

I'm so sorry. :( I've been wanting to update it for a long time. Thanks for the PR, I'll publish it asap.

[bendera]

I'm working on a new docs site, you can check it in the docs-new branch. I'll add it to an info block in every place where is related.