Fix weekly contributions script

[lf94]

There were 2 noticeable issues with the script:

1. We were trying to post massive markdown into a GitHub PR comment / body.
2. We were hitting GitHub too frequently with too heavy requests.

To reduce errors, I've changed the comment body to mention looking at Files changed tab, and added waits between requests, and reduced our repo request "chunks" from 25 to 2.

[lf94]

For what it's worth, you can test locally using act:

act -W .github/workflows/weekly-contributions.yml -s GITHUB_TOKEN="..."