Documentation: ACPI: move apei/output_format.txt to firmware-guide/acpi and convert to reST

This converts the plain text documentation to reStructuredText format
and adds it to Sphinx TOC tree.

No essential content change.

Signed-off-by: Changbin Du <changbin.du@gmail.com>
Reviewed-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

Documentation/firmware-guide/acpi/index.rst

@@ -17,6 +17,7 @@ ACPI Support
    DSD-properties-rules
    debug
    aml-debugger
+   apei/output_format
    gpio-properties
    i2c-muxes
    acpi-lid