docs: add managing tags page and environment tagging guide

[dangrondahl]

Summary

• Adds administration/managing_tags.md — full reference for tags including add/remove/read via Terraform, CLI, and API, recommended tag patterns (tier, team, region, risk-level, cost-center), and usage in environment policies
• Documents that URL tag values automatically render as clickable links in the UI
• Adds a "Tagging environments" section to getting_started/environments.md with a quick example and cross-link
• Adds the page to the Administration nav group in config/navigation.json

Closes #190

Test plan

• Verify administration/managing_tags renders correctly on local dev server
• Verify the "Tagging environments" section on getting_started/environments links correctly
• Confirm navigation shows the new page under Administration

[mintlify]

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Project	Status	Preview	Updated (UTC)
kosli	🟢 Ready	View Preview	Apr 30, 2026, 2:01 PM