docs(fix): reference octokit context and github event webhook payloads in variables

[zimeg]

Summary

This PR matches the source of the default context to what's included with octokit - highlighted in and fixes #347.

The README now references the octokit context and the webhook event payloads provided as a default, with the option to provide custom payloads. 🚀

Reviewers

Here's an example workflow run where this default context is logged - it matches what octokit provides 🙏

Notes

Overall we might want to include more details about how payloads are parsed in this "Sending Variables" section, but I think that'd be alright in following changes! 📚

Dreams might exist to make use of the repo Wiki for more examples and nuance, but this also might be better for follow up.

Requirements

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.91%. Comparing base (2c86117) to head (4e7f0ef).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #348   +/-   ##
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  Coverage   97.91%   97.91%           
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  Files           2        2           
  Lines          96       96           
=======================================
  Hits           94       94           
  Misses          2        2           

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[zimeg]

@filmaj Thank you for the quick review! ✅ 💨