Update ClickUp integration docs with Custom Task IDs information

Update clickup.md with clickup's customer support resolved issue solution from Live Chat Follow-up: Custom Task IDs in GitLab (#1711511)

This is the clickup representative message

This is because Custom Task IDs require your Workspace/team ID to be present as well.

 

After some further testing, I can confirm that if you use the format below, it will then generate links for both Custom Task IDs and normal task IDs:

https://app.clickup.com/t/WorkspaceID/:id

 

You need to replace the WorkspaceID part with your Workspace ID which you can get by simply logging in to your ClickUp account it's the numbers right after app.clickup.com, so as an example for mine:

Therefore, although the t/:id format still works, since the t/workspaceId/:id is more generic, I think it makes sense to just recommend that second one, instead of explaining the differences and when to use one vs the other.

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This is just documentation change with information supported directly from ClickUp, the 3rd party integration platform here.

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