Add custom executor documentation

Steve Xuerebrequested to merge
docs/4257-document-generic-executor into master

What does this MR do?

Add custom executor documentation

Todo

  • Document ExecTerminateTimeout & ForceKillPeriod and what they do.
  • Update document to remove generic executor wording and just name it exec
  • RunArgs are the first things passed to the executable which will be followed by the arguments GitLab Runner sends.

What are the relevant issue numbers?

#4257 (closed)

Custom Executor: !1385 (merged)

Edited by Steve Xuereb

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