Standardize location of example configs and other docs

[wileyj]

ex: ./testnet/stacks-node/conf

should be moved to a more standardized location like ./docs, ./contrib or a top-leval ./examples folder.

ref #5551

[wileyj]

This is growing a bit larger than i anticipated - the sample configs are used/referenced in other places that i'll need to dig into like ./docs/profiling.md

the basic idea i'm pursuing here is:
move testnet/stacks-node/conf -> sample/conf
mv sample-contracts -> sample/contracts

update all references in the repo to point to the new location (side note: in develop, testnet/stacks-node/conf is a symlink to ./stackslib/conf, i plan to link that in reverse so that stackslib has the symlink to ./sample/conf).