docs: document degu v0.1.5 as a release

[FeathBow]

Summary

Stabilizes the release-facing documentation so the v0.1.5 tag will not carry pre-release, self-contradictory wording. The README and the installation, usage, and safety guides all now state plainly that they document degu v0.1.5 and tell readers on a different binary to use the docs tagged for their version, instead of the previous "unreleased v0.1.5 / next release / latest published is v0.1.4" phrasing that would be permanently wrong once tagged (and, via readme = "../../README.md", would make the crates.io degu 0.1.5 page claim to be unreleased). Specifically:

• README banner rewritten to "This README documents degu v0.1.5 …"; removed "currently v0.1.4" from the install heading; dropped "On the next release" / "For main and the next release" / "The next release can …" so feature descriptions read in the present tense.
• The version-pinned v0.1.4 documentation links (which framed v0.1.4 as the current release) are replaced with version-matched relative links (docs/installation.md), so the tagged README points at the matching tag's guides rather than a hardcoded older version.
• docs/installation.md, docs/usage.md, and docs/safety.md banners rewritten the same way ("This guide documents degu v0.1.5 … use the guide tagged for your version").

No code, CLI, or behavior changes — documentation wording and links only.

Validation

• cargo fmt --all -- --check — clean.
• cargo test --workspace --all-features --locked — all green, including the byte-exact README contract test (the pinned scan-output block is unchanged).
• Documentation link check (lychee 0.24.2, the CI version, offline with --include-fragments) — 0 errors; the new relative links resolve to the local guides and the Tool-native reclaim fragment resolves against the local docs/usage.md.

Safety

None. This changes only documentation prose and links; no scan, clean, staging, undo, purge, quota, relocate, or path/symlink behavior changes, and no code is touched.