Add new Antlers Security section

[morhi]

Recent PRs introduced breaking changes and improvements to Antlers security.

I'd suggest to create new sections for

• an upgrade path at https://statamic.dev/getting-started/upgrade-guide
• explaining how Antlers Security works at https://statamic.dev/frontend/antlers

Topics to cover:

• Explain trusted/untrusted mode and its scope
• List of available trusted modifiers, tags
• Using config props
• Document Antlers::parser and things to be aware of
• Which CP fields are affected of the security restriction (like collection route fields)
• If projects modifiers and tags are trusted automatically
• How addons can register trusted modifiers and tags
• How to debug modifiers and tags that don't seem to work (return null/previous value silently)
• Things to be aware of when using view or other helpers to render Antlers templates from code

Related PRs:

• [5.x] Fix Antlers parser state restoration cms#14151
• [5.x] Antlers config backwards compatibility cms#14146
• [5.x] More Antlers defaults cms#14125
• [5.x] Antlers hardening cms#14092