[ruokun-niu]

Description

Expose the OTel Observability tutorial under Drasi Server → Tutorials.

The tutorial was merged in drasi-project/learning-drasi-server#10, but this
docs site explicitly mounts individual tutorials and ignores the upstream
module configuration. Fetching the latest upstream content therefore did
not make the new tutorial appear.

Changes

• Add an explicit mount for the OTel Observability tutorial, excluding its
  README and HTML files.
• Update the tutorial module pin to include the merged tutorial.
• Document the mount requirement for new tutorials and explain the
  difference between local and CI module versions.

Type of change

• This pull request is a minor refactor, code cleanup, test improvement, or other maintenance task and doesn't change the functionality of Drasi (issue link optional).

Fixes: #issue_number

[Copilot]

🟢 Approval recommended

The mount, module pin, and documentation are consistent, with no blocking issues identified.

No open issues 🎉

What changed in this PR

Adds the OTel Observability tutorial to the Drasi Server documentation navigation.

Changes:

• Mounts the tutorial explicitly, excluding README and HTML files.
• Updates the upstream tutorial module pin and checksums.
• Documents mount requirements and local versus CI module versions.

File	Description
readme.md	Explains tutorial mounts and module version selection.
docs/​go.sum	Updates tutorial module checksums.
docs/​go.mod	Pins the module version containing the tutorial.
docs/​config.toml	Mounts the OTel Observability tutorial.

_{Review effort: Balanced}