docs: Slack integration requires "chat:write" permissions scope

[jpuris]

SUMMARY

Slack requires chat:write as part of "OAuth & Permissions" for the slack integration to be able to post messages.
If the mentioned scope is not defined, one will receive following error from Slack API

Error: The request to the Slack API failed. The server responded with: {'ok': False, 'error': 'missing_scope', 'needed': 'chat:write:bot', 'provided': 'incoming-webhook,files:write'}

TESTING INSTRUCTIONS

To reproduce the problem

1. Add slack integration as per current instructions
2. Invite the app to the target slack channel
3. Schedule an alert to be posted on specific channel
4. Error. The request to the Slack API failed. The server responded with: {'ok': False, 'error': 'missing_scope', 'needed': 'chat:write:bot', 'provided': 'incoming-webhook,files:write'}

Testing the fix

1. Add slack integration as per current instructions, if already done, then skip to next step
2. Add additional scope chat:write
3. If step 2 was done after initial installation of slack app, then reinstall the slack app on workspace
4. Invite the app to the target slack channel
5. Schedule an alert to be posted on specific channel
6. Message is posted as expected

ADDITIONAL INFORMATION

• Has associated issue: Slack integration documentation is missing a permission scope #15903

[srinify]

Thanks @jpuris for adding this !

[villebro]

LGTM - big thanks for the PR! 🎉 👍