WIP: Add markdown doc for some examples

Open Müller, Felix requested to merge s5272799/amdis-core:feature/add_example_doc into master

Add doc for ellipt.cc, heat.cc, vecellipt.cc.

WIP: Requires !220 (merged).

see https://amdis-test.readthedocs.io/en/feature-add_example_doc/

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