add info about MaxSubmitJobs to Array jobs intro

[msainio]

Proposed changes

Add info about MaxSubmitJobs to Array jobs intro. Suggest xargs and GNU Parallel as alternatives to HyperQueue.

Preview page: https://csc-guide-preview.rahtiapp.fi/origin/array-job-task-limit/computing/running/array-jobs/

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• [ x] I have followed the instructions in the Contributing and Styleguide documents.
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