API docs standardization changes

[MikeAdamsMews]

Summary

Some small changes to documentation as part of API standardization initiative. These changes bring the API part of the Booking Engine Guide into line with the other Mews APIs. A couple of other necessary changes were made at the same time. Full list of changes as follows:

• API: Guidelines renamed to Usage guidelines
• API: Updated Deprecations page definition of discontinued
• API: Changed Whitelisting to IP address allowlisting
• API: Authorization page renamed to Authentication and content rewritten
• General: Added .gitbook.yaml file with redirect for old authorization page
• General: Google Tag Manager Configuration title and capitalization adjustments
• General: Migration off the mews.li domain title adjustment
• General: Rolled over changelog to 2025

Checklist (general)

• Documentation follows the Style Guide
• Changelog dated the day when PR merged
• Changelog accurately describes all changes
• All hyperlinks tested
• SUMMARY.md updated if new pages added

Checklist (API)

• JSON examples updated
• Properties in JSON examples in same order as in property tables
• Changelog highlights the affected endpoints or operations
• Changelog highlights any deprecations
• Deprecation Table updated if any deprecations