Doc: ABI: overhaul ABI report locations

Old variable | New variable | Old location | New location
-------------+--------------+------------------+-------------------
BUILD_ROOT | ABI_CHECKOUT | .ABI_build | abi/src
(none) | ABI_INSTALL | .ABI_build/root | abi/install
(none) | ABI_DUMPS | abi_dumps | abi/dumps
(none) | ABI_REPORTS | compat_reports | abi/compat_reports