Update `clicraft dl` docs
Update explore.sh docs
Change blocks.sh mode 755 -> 644
Add usage to blocks.sh
Fix indentation of usage for pregen.sh
Move all example actions into src/etc/action.d

The example actions in the scripts/ directory were placed there because,
unlike the examples in src/etc/action.d, they wrapped third-party software
and required that software to run. As more and more examples get added, it's
becoming clear that this distinction isn't useful, and makes documentation
more difficult. So put them alongside the other example scripts and install
them in the same way.

Also, remove the --with-mcexplore, --with-c10t, and --with-mmat options to
./configure, as they're no longer useful.
Add basic docs for example actions

Add a clicraft-examples(1) man page to document the *.sh.example
actions. Much of the documentation is lifted from the terse help
output and needs to be expanded.