docs: add more schedule sources

[danfimov]

No description provided.

[Copilot]

Pull Request Overview

This PR updates the schedule sources documentation by adding information about additional schedule sources and making structural improvements to the documentation organization.

• Reorganizes schedule sources into "Official" and "Third-party" categories
• Adds documentation for NATS schedule source
• Adds references to PostgreSQL and YDB schedule sources
• Cleans up code example formatting in the testing guide

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
docs/available-components/schedule-sources.md	Restructures documentation with new sections for official and third-party schedule sources, adds NATS/PostgreSQL/YDB sources
docs/guide/testing-taskiq.md	Removes unnecessary ellipsis placeholders from code example

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