Preserve link IDs when possible

[Beanow]

As a small readability feature when looking at the output markdown, it'd be nice if existing IDs in docs like:

[Have you a link][some-link-id]

[some-link-id]: https://github.com/msrd0/cargo-doc2readme

Were preserved and didn't turn into:

[Have you a link][__link0]

[__link0]: https://github.com/msrd0/cargo-doc2readme

It looks like this would depend on pulldown-cmark/pulldown-cmark#434
Which got resolved, but is not yet released in a new crate version (seems planned to be v0.10.0).
https://github.com/raphlinus/pulldown-cmark/blob/80eeca9ca87b55e5114db019b93b50aadc2a66cf/src/lib.rs#L187-L188

[msrd0]

It looks like this is blocked on a new release from pulldown-cmark.

[msrd0]

We are using pulldown-cmark >=0.10 for some time now. If someone wants to implement this, and the code changes required are minimal, I'm happy to review a PR. Otherwise, note that verbatim copy of your markdown is a non-goal of this project, since a readable version of the same markdown is already present, and this tool is only here to put that markdown somewhere websites like crates.io, codeberg.org, github.com etc can find and present it.