Use MicroCeph tag for docs build

Currently we pin a specific commit on MicroCeph's `main` branch for the combined docs build. We should rather use the upstream https://github.com/canonical/microceph/releases/tag/v19.2.0%2Bsquid tag instead as it gets updated and will itself resolve to MicroCeph's standalone `stable` docs.

Locate the configuration for the combined docs build and inspect how the MicroCeph main-branch commit is currently pinned. Change that source reference to the upstream v19.2.0+squid release tag, then run the repository's docs build to verify it resolves to the standalone stable docs and completes successfully.

Written by the indexing model from the issue text.