Docs: On-call schedule updates continued

[alyssawada]

PR #1197 introduced the first iteration to establish initial on-call schedule doc improvements.

This issue captures what's included in the second iteration of on-call schedule docs:

• Improve "About web-based schedules" doc to provide additional concept content
• Add section "Key concepts and features" to define and explain terminology.
• Improve "Import on-call schedules" doc to include everything captured in issue Docs: Improve iCal schedule setup #1078
• New doc: Manage and configure rotations
• Specific guidance for rotation configuration. Rotation configuration examples and explanations.
• Update linked docs to support on-call schedules
• User settings and permissions
• Escalation chains
• Improve user permissions callouts in schedule docs

[alyssawada]

Closing as stale in favor of new IRM doc work