Improve documentation for SLACK_THREAD_TS

[CODEBRAKERBOYY]

** Improve docs for SLACK_THREAD_TS

This PR adds clear examples for replying in Slack threads, fixing confusion in #210.

*** Changes

• Added new section to README
• Added examples for:
  1. Using Slack message URL
  2. Same job (API mode)
  3. Across different jobs (using job outputs)

*** Why

• Many users are confused about SLACK_THREAD_TS
• This makes it easy to copy-paste and use correctly

[CODEBRAKERBOYY]

Hi maintainers,
I’ve opened this PR to improve the documentation for SLACK_THREAD_TS.
All checks have passed successfully.
It would be great if you could take a look whenever you get a chance.
Thanks a lot for your time and support!
Best,
Alok