Documentation: actions items

[SimonHeybrock]

• Consider moving some technical documentation into the source tree, reducing risk of running out-of-data. See, e.g., a number of pages in https://scipp.github.io/reference/developer-documentation.html.
• In the source tree (mainly scipp), add basic docs what components do, i.e., overview pages of how various concepts are tied together.
• Add more examples to function docstrings.