fix: Fact-check of scheduling

[Swiftaxe]

Summary

Fact-checked, removed noise, and added frontmatter to 5 scheduling pages.

All 5 files:

• Added missing description frontmatter

01-setup:

• Removed stray import 'package:serverpod_auth_idp_server/core.dart'; from the server.dart example (unrelated auth import)

02-recurring-task:

• Fixed "continously" → "continuously"
• Removed same stray auth import and stripped trailing spaces from the code block

03-inheritance:

• "augment the superclass's one" → "overriding the superclass implementation"

04-configuration:

• Removed redundant "Below is an example of how you can configure future calls in a YAML file:" lead-in
• Removed 5× repeated "Example configuration:" labels
• Fixed checkBrokenCalls and deleteBrokenCalls examples — they were shown at the top level but should be nested under futureCall: per the configuration reference (factual fix)

05-legacy:

• Removed "is simple" filler
• Trimmed verbose registration paragraph
• "You are now able to register a future call to be invoked in the future by calling either..." → "With the future call registered, you can schedule it using either..."
• DateTime(2025, 1, 1) → DateTime(2030, 1, 1) (past date)
• Fixed note text (lowercase start, unclear reference to "model folder")