docs(release): point distribution status at Maven Central + javadoc.io (H3)

[DemchaAV]

Summary

Wires up Track H3 — hosted Javadocs. javadoc.io auto-mirrors any artefact published to Maven Central within minutes, so no separate hosting infrastructure is required. This PR is just the README documentation step pointing callers at the URL.

README distribution-status note

-> **Distribution status** — currently **JitPack**. Maven Central is planned for v1.7 ([tracking issue](https://github.com/DemchaAV/GraphCompose/issues/7)).
+> **Distribution status** — currently **JitPack**. **Maven Central**
+> ships from **v1.6.6** under coordinates `io.github.demchaav:graphcompose:<version>`;
+> hosted Javadocs auto-publish to [javadoc.io/doc/io.github.demchaav/graphcompose](https://javadoc.io/doc/io.github.demchaav/graphcompose) shortly after each Central release.
+> JitPack stays available alongside Central for existing callers.

The old line claimed Central was planned for v1.7 — outdated once the D-track (#95 / #96 / #97 / #98) shipped the v1.6.6 publish pipeline.

What this doesn't do

• Does not change the JitPack install snippet. That stays as-is — cut-release.ps1 rewrites it to v1.6.6 at release time, and VersionConsistencyGuardTest validates the post-bump state. Adding a Central snippet now would require either pinning a future version (snippet pointing at a tag that doesn't exist breaks JitPack-style copy-paste) or duplicating the guard logic; both are friction without payoff yet.
• Does not add a "Central as primary, JitPack as fallback" snippet. That's the v1.6.6 release-prep PR's job — explicitly listed in the readiness taskboard — after the first Central publish proves the pipeline end-to-end. This PR is the lightweight documentation step that's safe to ship now.

Verification

$ ./mvnw -B -ntp test -pl . -Dtest='CanonicalSurfaceGuardTest,DocumentationCoverageTest,VersionConsistencyGuardTest'
Tests run: 13, Failures: 0, Errors: 0, Skipped: 0

CHANGELOG entry added to v1.6.6 — Planned under ### Build.

Test plan

• Doc guards green locally
• CI green on PR
• Reviewer skim — single sentence change in README + matching CHANGELOG entry