Remove unused cross-arch files and docs (#547)

Cross-arch tests now run in their own native ARM runner in GitHub, so
these files are not used, they are not validate and could be broken.

Users that want to test on a different arch than the one they develop on
would have to use a custom setup.

Author: llucax

.github/RELEASE_NOTES.template.md

@@ -13,7 +13,7 @@
 All upgrading should be done via the migration script or regenerating the templates.
 
 ```bash
-curl -sSL https://raw.githubusercontent.com/frequenz-floss/frequenz-repo-config-python/<tag>/cookiecutter/migrate.py | python3
+curl -sSLf https://raw.githubusercontent.com/frequenz-floss/frequenz-repo-config-python/<tag>/cookiecutter/migrate.py | python3 -I
 ```
 
 But you might still need to adapt your code:

.github/containers/nox-cross-arch/arm64-ubuntu-20.04-python-3.11.Dockerfile

@@ -21,7 +21,7 @@
 """  # noqa: E501
 
 # R0801 is similarity detection, as the template is always similar to the current script
-# pylint: disable=too-many-lines, too-many-locals, too-many-branches, R0801
+# pylint: disable=too-many-lines, too-many-locals, too-many-branches, too-many-statements, R0801
 
 import hashlib
 import json

.github/containers/nox-cross-arch/entrypoint.bash

@@ -261,30 +261,3 @@ These are the steps to create a new release:
    should be automated eventually too).
 
 7. Celebrate!
-
-##  Cross-Arch Testing
-
-This project has built-in support for testing across multiple architectures.
-Currently, our CI conducts tests on `arm64` machines using QEMU emulation. We
-also have the flexibility to expand this support to include additional
-architectures in the future.
-
-This project contains Dockerfiles that can be used in the CI to test the
-python package in non-native machine architectures, e.g., `arm64`. The
-Dockerfiles exist in the directory `.github/containers/nox-cross-arch`, and
-follow a naming scheme so that they can be easily used in build matrices in the
-CI, in `nox-cross-arch` job. The naming scheme is:
-
-```
-<arch>-<os>-python-<python-version>.Dockerfile
-```
-
-E.g.,
-
-```
-arm64-ubuntu-20.04-python-3.11.Dockerfile
-```
-
-If a Dockerfile for your desired target architecture, OS, and python version
-does not exist here, please add one before proceeding to add your options to
-the test matrix.

.github/containers/test-installation/Dockerfile

@@ -2,10 +2,12 @@
 
 ## Summary
 
-This release improves workflows security, adds a black migration workflow, and fixes failed migrations from version v0.16.0.
+<!-- Here goes a general summary of what this release is about -->
 
 ## Upgrading
 
+<!-- Here goes notes on how to upgrade from previous versions, including deprecations and what they should be replaced with -->
+
 ### Cookiecutter template
 
 All upgrading should be done via the migration script or regenerating the templates.
@@ -16,16 +18,20 @@ curl -sSLf https://raw.githubusercontent.com/frequenz-floss/frequenz-repo-config
 
 But you might still need to adapt your code:
 
+<!-- Here upgrade steps for cookiecutter specifically -->
+
 ## New Features
 
+<!-- Here goes the main new features and examples or instructions on how to use them -->
+
 ### Cookiecutter template
 
-- Add a `black-migration.yaml` workflow that automatically reformats code when Dependabot upgrades `black`.
+<!-- Here new features for cookiecutter specifically -->
 
 ## Bug Fixes
 
+<!-- Here goes notable bug fixes that are worth a special mention or explanation -->
+
 ### Cookiecutter template
 
-- Fix migration of CI workflow matrices that used `arch`/`os` dimensions with values different from the default template. The v0.16.0 migration relied on exact string matching, so projects with customized matrix items (for example `arch: [amd64]`, `os: [ubuntu-24.04]`) could be left only partially migrated. The new migration step rebuilds the `platform` entries from the existing `arch`/`os` values and only rewrites `runs-on` when it still points to the old matrix keys.
-- Improve workflows security: tighten permissions, avoid potential shell injection, run Python in isolated mode, pin all dependencies using the SHA hash.
 - The unused cross-arch QEMU-based testing infrastructure has been removed. The `.github/containers/nox-cross-arch/` and `.github/containers/test-installation/` directories, as well as the "Cross-Arch Testing" section in `CONTRIBUTING.md`.