ci: add docs.rs compatibility checks

[par1ram]

Summary

• add a GitHub Actions workflow that checks docs.rs compatibility for all seven published application crates
• prefetch dependencies, then run cargo docs-rs --offline inside an isolated Linux network namespace
• document the reusable docs-rs-check.sh helper

Why

docs.rs builds crates without network access. A regular documentation build can therefore miss regressions where a dependency or build script attempts a download, as happened in #519.

This adds a focused regression check while keeping the implementation smaller than running the full docs.rs infrastructure.

Closes #524.

Validation

• cargo +nightly docs-rs --offline passed for:
  • stratum-apps
  • pool_sv2
  • jd_server_sv2
  • jd_client_sv2
  • translator_sv2
  • integration_tests_sv2
  • bitcoin_core_sv2
• actionlint .github/workflows/docs-rs.yaml
• shellcheck scripts/docs-rs-check.sh
• bash -n scripts/docs-rs-check.sh
• git diff --check

The full Linux network-namespace path is exercised by the new workflow on this pull request.

[bit-aloo]

We don't wanna use sudo on ci

[plebhash]

@par1ram on this project we try to keep a reasonable level of git hygiene

if a reviewer asks you to change something, please don't add a new commit pivoting commit history to a new direction... simply rebase the original commit while atomically incorporating the review

in other words: ecd98fa shouldn't exist as a standalone commit