Separate section for detailed technologies description

To keep individual sections concise/to prevent over-cluttering throughout the documentation, while still having enough detail, it might be worthwhile adding a separate section that features an overview with more detailed technology descriptions.
(Previously proposed e.g. [here](https://github.com/iiasa/message_doc/pull/24/#issuecomment-557005769))

Start by reviewing the existing documentation sections and the proposal in pull request 24's referenced comment. Define the separate overview section and confirm that individual sections remain concise while the new section provides detailed technology descriptions.

Written by the indexing model from the issue text.