Adds Fumadocs coverage for the Usage feature

[MSami625]

What Changed?

Adds Fumadocs coverage for the Usage feature:

• New page: docs-fumadocs/content/docs/monitoring/usage.mdx — overview, metrics, filters, drill-down, data freshness/history, pricing overrides, and ops notes
• Adds Usage as the first item under Monitoring in the sidebar (monitoring/meta.json)
• Links Usage from intro.mdx (Core capabilities + Key guides)
• Adds Usage pricing CLI section to reference/cli-commands.mdx (seed-rates, diff-rates, recompute, sync-litellm, env tuning)
• Updates feature-contributors.json and search index

No application code changes in this commit.

Why?

The Usage feature was implemented in the app but had no user-facing documentation in Fumadocs. This fills that gap so reviewers and customers can understand how Usage works (metrics, drill-down, entitlements, pricing overrides, and ops) in the same style as other docs (Metrics, Calls, etc.).

How to Test?

cd docs-fumadocs && npm run ci:check

Then locally:

npm run dev

1. Open http://localhost:3000/docs/monitoring/usage/
2. Confirm Usage appears first under Monitoring in the sidebar
3. Spot-check internal links (workspaces, integrations, configuration, CLI commands)
4. Open /docs/intro/ and confirm the Usage link under Key guides
5. Open /docs/reference/cli-commands/ and confirm the Usage pricing section

Release Label

• major - breaking change, next release bumps major version
• minor - backward-compatible feature, next release bumps minor version
• fix - backward-compatible bug fix, next release bumps patch version
• No label (defaults to patch release)

If you do not have permission to apply labels, mention the intended release label here and a maintainer will set it.

Checklist

• [ x] I have read the CONTRIBUTING.md guide.
• [ x] My code follows the project's style guidelines.
• [ x] I have added tests that prove my fix is effective or my feature works.
• [ x] I have updated documentation where needed.

[greptile-apps]

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