Add documentation about config for java heap in API Mediation Layer services

[pablocarle]

Describe your pull request here:

This PR adds an article about advanced configuration for the API Mediation Layer Java-based services.

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[janan07]

Perhaps it would be useful to describe what "heap memory size" is...

[pablocarle]

Ok, I'll give it a try

[janan07]

Reviewed and edited by doc squad

[janan07]

Looks good

[balhar-jakub]

It looks good, but should be against master branch as it applies to version 2.15 as well.