Document k6 cloud file size limits

[immavalls]

Based on https://community.k6.io/t/getting-error-unexpected-http-error-from-https-ingest-k6-io-v1-archive-upload-413-request-entity-too-large-when-running-test-on-k6-cloud-with-large-json-files-and-160-users/5983, k6 cloud has a limit of 100Mb to upload files (client_max_body_size 100M). Which I could not find documented. We should probably add this to the cloud docs.