docs: include versioning details and migration guides in the readme

[zimeg]

Summary

This PR adds a migration guide to the README and a "table of contents" to find fast links between section headers 👾 ✨

Following #398!

Preview

🔗 https://github.com/slackapi/slack-github-action/tree/docs-migration-guide

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 99.85%. Comparing base (f841ea5) to head (aecc23f).
Report is 1 commits behind head on main.

Additional details and impacted files

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  Coverage   99.85%   99.85%           
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  Files           7        7           
  Lines         702      702           
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  Hits          701      701           
  Misses          1        1           

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[WilliamBergamin]

Nice 🚀 great job!!

[zimeg]

@WilliamBergamin Thanks a ton! But all credit to @thejeff77 for suggesting that this info be included in the README.md 👾

I'll go ahead with a merge now, but I'm so open to revisiting how or even where this information is organized later 📚 ✨

[thejeff77]

Thanks folks!

I have some additional tidbits to share, unfortunately with a lot of the inputs moving to the payload, actionlint is much less helpful in verifying if the action is still valid after migration. Requires a lot of trial and error.

It would be pretty cool if chatGPT or Copilot could be trained up on migrations to make this a bit more automated. I'm sure it will get there at some point anyway.

Hints on checking validity locally without having to commit, push, run would be welcome! (If they exist)