contrib/plugins/uftrace: add documentation

contrib/plugins/uftrace: add documentation

This documentation summarizes how to use the plugin, and present two
examples of the possibilities offered by it, in system and user mode.

As well, it explains how to rebuild and reproduce those examples.

Reviewed-by: Manos Pitsidianakis <manos.pitsidianakis@linaro.org>
Signed-off-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Message-ID: <20250902075042.223990-10-pierrick.bouvier@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-ID: <20250922093711.2768983-26-alex.bennee@linaro.org>