Auto merge of #11480 - arlosi:registrydocs, r=ehuss

Split up registry documentation into multiple sections

Splits up the documentation for registries into multiple sections. Avoids making any changes to the docs themselves beyond adding links.

The upcoming stabilization of the sparse registry protocol will include more content changes to these docs.

Closes #11476
r? `@ehuss`

Author: bors

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)