Rollup merge of #149565 - jyn514:force-doctest-merge, r=notriddle

rustdoc: Add unstable `--merge-doctests=yes/no/auto` flag

This is useful for changing the *default* for whether doctests are merged or not. Currently, that default is solely controlled by `edition = 2024`, which adds a high switching cost to get doctest merging. This flag allows opting in even on earlier editions.

Unlike the `edition = 2024` default, `--merge-doctests=yes` gives a hard error if merging fails instead of falling back to running standalone tests. The user has explicitly said they want merging, so we shouldn't silently do something else.

`--merge-doctests=auto` is equivalent to the current 2024 edition behavior, but available on earlier editions.

Helps with rust-lang/rust#141240. ``@epage`` said in that issue he would like a per-doctest opt-in, and that seems useful to me in addition to this flag, but I think it's a separate use case from changing the default.

Author: matthiaskrgr