Split up registry documentation into multiple sections

Author: arlosi

src/doc/src/SUMMARY.md

@@ -36,6 +36,9 @@
     * [Source Replacement](reference/source-replacement.md)
     * [External Tools](reference/external-tools.md)
     * [Registries](reference/registries.md)
+        * [Running a Registry](reference/running-a-registry.md)
+            * [Registry Index](reference/registry-index.md)
+            * [Registry Web API](reference/registry-web-api.md)
     * [Dependency Resolution](reference/resolver.md)
     * [SemVer Compatibility](reference/semver.md)
     * [Future incompat report](reference/future-incompat-report.md)