Automatically add a py.typed file alongside stubs

[lagru]

Release note

For maintainers and optionally contributors, please refer to changelist's README on how to document this PR for the release notes.

Automatically add a `py.typed` file alongside generated stubs.
This ensures that type checkers can make use of the stubs.
{label="enhancement"}

Add a short section on "Distributing stubs files" to the
introduction guide. This also describes docstubs behavior 
regarding `py.typed` files. 
{label="documentation"}

Prevent `.github/scripts/assert-unchanged.sh` from exiting early.
{label="devops"}