Adding documentation for the signin command (#42223)

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
Co-authored-by: Sharra-writes <sharra-writes@github.com>

Author: 3 people

content/integrations/how-tos/slack/use-github-in-slack.md

@@ -37,6 +37,7 @@ To use a slash command, type `/github` followed by the command you want to execu
 |`/github close [issue link]`|Closes the specified issue as completed.|
 |`/github close [issue link] reason:"not planned"`|Closes the specified issue with a reason. Replace `"not planned"` with your reason.|
 |`/github reopen [issue link]`|Reopens the specified issue.|
+|`/github signin`|Restarts the "Connect your GitHub account" workflow.|
 
 >[!NOTE] When you subscribe a channel to a repository, the channel will receive notifications for all `open`, `close`, and `reopen` events on pull requests and issues in that repository. The channel will also receive notifications of any `push` events directly to the repository's default branch.