[#65] Allow using the same position when defining `DocItem`

Description

Problem: When different contracts define custom doc items with the same DocItemPosition, then using those contracts in the same code won't be possible.

Solution: Make DocItemPosition into a plain method, and consider the name of the doc item as a part of the ordering as well by using (docItemPos, docName) instead of just the docItemPos.

Related issue(s)

Resolves #65 (closed)

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