Adding multientity to tutorials

[tstorek]

Is your feature request related to a problem? Please describe.
Currently, there is neither a section in the tutorial nor an example on how to leverage registrations or the multi-entity plugin.

Describe the solution you'd like
Adding an additonal tutorial section that explains the multi-entity functionalities in the iot agent. Here comes the expression language into play.

Additional context

• https://iotagent-node-lib.readthedocs.io/en/2.12.0/advanced-topics/index.html#multientity-plugin-multientity
• https://iotagent-node-lib.readthedocs.io/en/2.12.0/expressionLanguage/index.html#measurement-transformation-expression-language
• https://iotagent-node-lib.readthedocs.io/en/2.23.0/advanced-topics.html#multientity-plugin-multientity
• https://iotagent-node-lib.readthedocs.io/en/2.23.0/expressionLanguage.html#jexl-based-transformations

[github-actions]

Branch 164-Adding-multientity-to-tutorials created!