Promote problem matcher documentation to docs.github.com

[kojiromike]

I was quite baffled by how many linters seemed to be dropping support for specific GitHub output styles. After some research, I discovered problem matchers. I don't think I should have had to stray from docs.github.com to learn that annotations can be formed from something besides workflow commands, like echo "::error file=app.js,line=1::Missing semicolon".

While the docs do briefly mention actions/toolkit, it is only in the context of writing a custom action in JavaScript. But that's not what I'm trying to do. I'm just running something like run: eslint … in a workflow file, and didn't have a sensible way to connect that after doing setup-node that eslint --format stylish would be matched by an implicit problem matcher.

If there's some way to make this clear to folks reading https://docs.github.com/en/actions/reference/workflows-and-actions/expressions, thank you. If not, maybe someone like me will save time by stumbling across this issue.