Build the docs in CI for all PRs touching the mypy/ directory (#16068)

1. #16061 added a new error code, but didn't add any docs for the new
error code
2. Because nothing in the `docs/` directory was modified, the docs CI
job didn't run on that PR
3. Now the docs build is failing on `master` because we have an error
code without any documentation:
https://github.com/python/mypy/actions/runs/6112378542/job/16589719563

Author: AlexWaygood

.github/workflows/docs.yml

@@ -8,6 +8,10 @@ on:
   pull_request:
     paths:
     - 'docs/**'
+    # We now have a docs check that fails if any error codes don't have documentation,
+    # so it's important to do the docs build on all PRs touching mypy/errorcodes.py
+    # in case somebody's adding a new error code without any docs
+    - 'mypy/errorcodes.py'
     - 'mypyc/doc/**'
     - '**/*.rst'
     - '**/*.md'