Replace docs.rs build doc_auto_cfg feature with doc_cfg

[tnull]

Unfortunately, doc_auto_cfg was removed, breaking doc builds for v0.7.0-rc.0. Here we replace it with the doc_cfg attribute.

[ldk-reviews-bot]

👋 Thanks for assigning @tankyleo as a reviewer!
I'll wait for their review and will help manage the review process.
Once they submit their review, I'll check if a second reviewer would be helpful.

[tankyleo]

Looked up docs.rs docs, thought you might want to add this to CI somewhere instead of the script we've got in LDK:
https://docs.rs/about/builds#testing-builds-ci
https://github.com/dtolnay/cargo-docs-rs

[tnull]

Looked up docs.rs docs, thought you might want to add this to CI somewhere instead of the script we've got in LDK: https://docs.rs/about/builds#testing-builds-ci https://github.com/dtolnay/cargo-docs-rs

Yeah, maybe that would be worth. For now going ahead landing this.