Add custom parameters benchmark page

[Naarcha-AWS]

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[gkamat]

The sections for Parameter Sources and Custom Runners should be completely separate, since they pertain to entirely different items. Also, the examples should be re-written, to make them easier to understand.

We can provide new examples, but for now, the existing text should suffice. Separating the sections can perhaps be done as part of the current PR.

[Naarcha-AWS]

@gkamat: I separated both files. Let me know what additional modifications you would like me to make.

[Naarcha-AWS]

For adding support for an operation that doesn't exist in OSB already.

[Naarcha-AWS]

Suggested change

	A **parameter source** is a source outside of OpenSearch Benchmark that provides parameters to an operation.
	A **parameter source** is a source outside of OpenSearch Benchmark that provides parameters to an operation. You can add parameters to an existing operation in OpenSearch Benchmark or add custom parameters to a [runner]({{site.url}}{{site.baseurl}}/benchmark/user-guide/optimizing-benchmarks/custom-runners/).