Change platform webserver cache control enabled default to true

[kvelarde-itential]

Changed the platform_webserver_cache_control_enabled default to 'true'. Previously it was unset.

Updated three places to reflect platform_webserver_cache_control_enabled now defaulting to true in the deployer, and to note that Itential Platform's own application default (when unset in platform.properties) is false:

• roles/platform/defaults/main/webserver.yml — expanded the comment above the var to state the override explicitly.
• roles/platform/CLAUDE.md — updated the webserver.yml defaults table row (default now true, purpose column notes the app's own default is false).
• docs/itential_platform_guide.md — updated the Webserver Variables table row the same way.