docs: fix docs.rs build

[00xc]

Summary of the PR

The current documentation build in docs.rs breaks with the following error:

[INFO] [stderr] error[E0557]: feature has been removed
[INFO] [stderr]   --> src/lib.rs:11:29
[INFO] [stderr]    |
[INFO] [stderr] 11 | #![cfg_attr(docsrs, feature(doc_auto_cfg))]
[INFO] [stderr]    |                             ^^^^^^^^^^^^ feature has been removed
[INFO] [stderr]    |
[INFO] [stderr]    = note: removed in 1.92.0; see <https://github.com/rust-lang/rust/pull/138907> for more information
[INFO] [stderr]    = note: merged into `doc_cfg`

So replace doc_auto_cfg with doc_cfg, which generates the expected docs, showing the required features for each module.

Requirements

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requirements:

• All commits in this PR have Signed-Off-By trailers (with
  git commit -s), and the commit message has max 60 characters for the
  summary and max 75 characters for each description line.
• N/A All added/changed functionality has a corresponding unit/integration
  test.
• All added/changed public-facing functionality has entries in the "Upcoming
  Release" section of CHANGELOG.md (if no such section exists, please create one).
• Any newly added unsafe code is properly documented.