Add a summary with a key

[jsoref]

Reading the log from [email protected] in https://github.com/check-spelling-sandbox/opentelemetry-collector-contrib/actions/runs/12681692584/job/35345929645 is frustrating. There's no output at the top indicating what to look for for errors and no output at the bottom.

Errors appear to be output like this, but it's really not easy to find:

[✖] https://library.humio.com/humio-server/api-ingest.html → Status: 403

Adding a small key would be immensely helpful to the average drive-by contributor who trips on this product's output while trying to do something totally unrelated.

That could be something like:

count files checked
count errors (indicated by [✖])
count working links (indicated by [✓])