Add documentation for tags

[dangrondahl]

Description

The current documentation does not cover tags at all. Tags are a Kosli feature (the CLI has kosli tag, and the API has a PATCH /tags/{org}/{resource_type}/{resource_id} endpoint), but there is no documentation explaining what tags are, how to use them, or what resources support them.

Proposed scope

• What are tags and why use them
• Which Kosli resources support tags
• How to add/update/remove tags via CLI (kosli tag)
• How to add/update/remove tags via API
• Practical use cases (organizing resources, filtering, etc.)

Location

Placement TBD — could be:

• A new page under Getting Started or Administration
• A section within an existing page

Related resources

• CLI: kosli tag (already in client reference at client_reference/kosli_tag)
• API: PATCH /tags/{org}/{resource_type}/{resource_id} (already in API reference)