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" <[email protected]>
Cc: Paolo Bonzini <[email protected]>
Cc: Maydell Peter <[email protected]>
Cc: John Snow <[email protected]>
Cc: Thomas Huth <[email protected]>
Cc: Alex Bennée <[email protected]>
Cc: Igor Mammedov <[email protected]>
Cc: Michael Tsirkin <[email protected]>
Signed-off-by: Ani Sinha <[email protected]>
Message-Id: <20221021095108[email protected]>
Reviewed-by: Michael S. Tsirkin <[email protected]>
Signed-off-by: Michael S. Tsirkin <[email protected]>