doc: Split and enhance subsections on "dev-review" and "TSC" labels

Split the explanation on dev-review and TSC labels into two separate ones,
and slightly reword both. For both, amend corresponding link to point to
the more relevant place in the online page on Zephyr meetings. Add
explanation for acronym "TSC".

Signed-off-by: Aleksandar Markovic <aleksandar.markovic.sa@gmail.com>