Explain URL uniqueness requirement for page URLs

[jdiehl]

Adds a new 'URL Uniqueness Requirement' section to the Page Properties reference page (refguide and refguide10).

The section explains:

• The constraint: URL patterns are compared structurally, so two pages with the same pattern but different parameter names will produce a consistency error
• The reason: the runtime must resolve which page to open from the URL alone, before loading any data — database queries during routing are impractical and would hurt performance
• Two workarounds: distinct static URL segments, and a microflow URL router