Clarify GitLab Observability tier and self-managed setup prerequisites (#146)

[simlarsen]

Resolves a reader's confusion (#146) about the GitLab CI OpenTelemetry post.

Current GitLab docs (18.1+) list CI/CD pipeline telemetry and GitLab Observability as Free/Premium/Ultimate with Experiment status, so the feature is not Ultimate-only. The older 16.x/17.x implementation that required Ultimate and the observability_features (formerly observability_tracing) flag was reworked, which is the source of the "Ultimate only" impression.

Changes:

• Added a "Tier and Availability" note stating the current tier/status and explaining the historical Ultimate-only confusion.
• Added a "Prerequisite: Set Up GitLab Observability First" section covering the GitLab.com UI enablement path and the self-managed backend + Rails console (Observability::GroupO11ySetting) path, clarifying that you do not enable it by editing gitlab.rb.
• Made the enablement step depend on that prerequisite and added a "no data" troubleshooting note.

Sources: https://docs.gitlab.com/operations/observability/ci_cd/ , https://docs.gitlab.com/operations/observability/observability/ , https://docs.gitlab.com/operations/observability/setup_gitlab_com/ , https://docs.gitlab.com/operations/observability/setup_self_managed/

npm run validate passes.

Resolves #146