Auto merge of #129337 - EtomicBomb:rfc, r=notriddle

rustdoc rfc#3662 changes under unstable flags

* All new functionality is under unstable options
* Adds `--merge=shared|none|finalize` flags
* Adds `--parts-out-dir=<crate specific directory>` for `--merge=none`
to write cross-crate info file for a single crate
* Adds `--include-parts-dir=<previously specified directory>` for
`--merge=finalize` to write cross-crate info files
* `tests/rustdoc/` tests for the new flags

Author: bors