Add executors documentation

[dimitraka]

Added the documentation in our manual. I think it would be nice to add some figures from the paper for the LULESH use case, but just wanted to check with you before I do so.

[hkaiser]

Added the documentation in our manual. I think it would be nice to add some figures from the paper for the LULESH use case, but just wanted to check with you before I do so.

Yes, that's definitely a good idea.

[hkaiser]

Could you add a small example that shows how to use a custom executor with a parallel algorithm (for completeness), please?

[codacy-production]

Coverage summary from Codacy

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Coverage variation	Diff coverage
Report missing for c0dab021	✅ ∅

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (c0dab02)	Report Missing	Report Missing	Report Missing
Head commit (ce55a63)	194974	166023	85.15%

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Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#6784)	0	0	∅ (not applicable)

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