Add page on serial execution

[rosieyohannan]

Description

Create overview page for serial execution across an org

Reasons

New feature

Content Checklist

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[liamclarkedev]

Some additional sections that might be worth mentioning around controlling these groups:

• Cancelling a workflow via the UI whilst a serial group is running will release the group for any waiting.
• Cancelling a workflow or "serial start"/lock job via the API will do the same.
• Clicking on a "serial start"/lock job node in the UI whilst a job inside the serial group is running will provide a dialog to release the group.

[liamclarkedev]

~~ Should this be key icon? Similar mentions below, but I won't comment on all of them.

Suggested change

	* A job in a serial group is preceded by a block with a lock icon to show the start of the serial group. A serial group end block follows the job.
	* A job in a serial group is preceded by a block with a key icon to show the start of the serial group. A serial group end block follows the job.

[rosieyohannan]

Oh yes good spot! Thanks