doc: samples: expose JSON-LD markup & meta description for code samples

Since samples are starting to leverage the new :zephyr:code-sample
directive, let's use the available  meta data to expose that structured
information to search engines.

This commit adds JSON-LD markup to the HTML output of code sample pages
and sets the meta description to the samples' description.

Signed-off-by: Benjamin Cabé <benjamin@zephyrproject.org>