Small number of doc comments in the project

[laruh]

The code base of the project has grown a lot over the years, so I think it's worth covering the project with doc comments to be able to generate a more detailed code documentation.

To check for doc comments, we can add missing_docs lint in CI. It detects missing documentation for public items.

There is also clippy linter missing_docs_in_private_items which warns for all items. The more comments the better, but this approach can have a negative impact on development speed.

First we need to check how long it takes to add doc comments for public items.