acpi/tests/avocado/bits/doc: add a doc file to describe the acpi bits test

acpi/tests/avocado/bits/doc: add a doc file to describe the acpi bits test

A doc file is added under docs/devel that describes the purpose of the various
test files and gives guidance to developers on where and how to make changes.

Cc: Daniel P. Berrange" <berrange@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Maydell Peter <peter.maydell@linaro.org>
Cc: John Snow <jsnow@redhat.com>
Cc: Thomas Huth <thuth@redhat.com>
Cc: Alex Bennée <alex.bennee@linaro.org>
Cc: Igor Mammedov <imammedo@redhat.com>
Cc: Michael Tsirkin <mst@redhat.com>
Signed-off-by: Ani Sinha <ani@anisinha.ca>
Message-Id: <20221021095108.104843-7-ani@anisinha.ca>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>