Documentation: pldmfw: Demote library overview section

Documentation: pldmfw: Demote library overview section

pldmfw library overview section is formatted as title heading (the
second title of index.rst), making it listed in driver-api toctree.

Demote the section.

Signed-off-by: Bagas Sanjaya <bagasdotme@gmail.com>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Tested-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Jacob Keller <jacob.e.keller@intel.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Message-ID: <20251103030228.23851-1-bagasdotme@gmail.com>