RFC: use validation actions

Currently the install instructions say to add

> --output_groups=+mypy

I used to have this requirement in rules_lint too, but changed to use validation actions instead:
https://github.com/aspect-build/rules_lint/pull/562

This is nicer IMO because:

1. registering the `--aspect` is sufficient to ask for the type-checker to run
2. users can still opt-out with Bazel flag `--norun_validations`
3. Removes a potential install mistake where the aspect is registered but its outputs weren't requested

Start by locating the install instructions that currently require `--output_groups=+mypy` and the Bazel aspect registration described in the issue. Compare the validation-action approach in rules_lint PR 562, then verify that registering `--aspect` runs the type checker without the output-group flag and that `--norun_validations` opts out.

Written by the indexing model from the issue text.