The new sub-command writes a string representation of the
current log level to the output variable given to the
sub-command.
Given that the log-level might be set either via the --log-level
command line option or via the CMAKE_MESSAGE_LOG_LEVEL
cache / regular variables, the priority for each of the log level
sources is as follows, with the first one being the highest:
1) --log-level
2) CMAKE_MESSAGE_LOG_LEVEL regular variable
3) CMAKE_MESSAGE_LOG_LEVEL cache variable
4) default log level (STATUS)
Fixes: #23572
The guide previously only focused on the find_package() command,
with a bias towards libraries. FetchContent was not mentioned at all.
Reorganise and update the existing content. Add new sections to cover
providing dependencies with FetchContent and dependency providers.
Improve discoverability of the guide by mentioning it at the beginning
of the find_package(), FetchContent and dependency provider docs.
