fix(ci): resolve docs build failure and gitleaks false positive

[daudee215]

Summary

Fixes two CI failures on main: broken docs build and a gitleaks false positive.

1. Docs build (mkdocs build --strict)

docs/changelog.md: The link [ROADMAP.md](../ROADMAP.md) used a relative path pointing outside docs_dir. mkdocs --strict mode rejects out-of-tree relative links.

Fixed by replacing with the absolute GitHub URL:

[ROADMAP.md](https://github.com/daudee215/line-noder/blob/main/ROADMAP.md)

2. Gitleaks secret scan

Gitleaks flagged a false positive sidekiq-secret match in README.md at line 42 (fingerprint: README.md:sidekiq-secret:42). No actual secret is present — the pattern matches on coincidental content.

Fixed by adding .gitleaksignore with the exact fingerprint to allowlist this known false positive.