feat: enhance github action summary with detailed repository updates

[chrheg]

This pull request introduces functionality to generate and append a job summary to a GitHub step summary file. The changes include updates to the evergreen.py script to create and append the summary, as well as modifications to the README.md to document the new job step. Additionally, new tests have been added to ensure the correctness of the new functionality.

Summary of Changes

Enhancements to evergreen.py:

• Added code to generate a job summary with details such as organization, follow-up type, dry run status, security updates, and project ID. The summary also includes a table of updated repositories. [1] [2] [3] [4]
• Introduced a new function append_to_github_summary to append the generated summary content to a specified file.

Documentation Updates:

• Updated README.md to include a new job step for posting the evergreen job summary to the GitHub step summary file. [1] [2] [3]

Testing:

• Added a new test class TestAppendToGithubSummary in test_evergreen.py to verify the functionality of the append_to_github_summary function.

[jmeridth]

@chrheg I love this idea. I'm going to test in a personal repo. And you have tests 🎉. Thank you.