Use a line length of 120

What should be done?

Using a line length of 120 for both code and documentation works great with typical sizes of our monitors, fonts, and tools:

• size 14 works great on a 14" laptop screen with a single sidebar;
• the same size on a bigger screen allows a second sidebar on the other side or multiple files to be opened side-by-side.

For documentation, we should insert newlines at the right places (see https://sembr.org). So we should not use reformatter that just wraps lines at random places.

Where?

Everywhere, both code and documentation.

Definition of Done

• all linters enforce line length 120;
• code and documentation updated;
• CONTRIBUTING.md updated;
• writing guides updated.

Don't do everything in one huge PR! Update some slices of files at once.