feat(gateway): CEL conditions for plugins

[lena-larionova]

Description

3.15 replaces plugin expressions in ATC with CEL.

Note: The file app/gateway/plugins/expressions-314.md does not need review, other than for the banner at the top. This is the old doc, renamed. I might turn this into a versioned page.

Fixes #5400

Preview Links

https://deploy-preview-5449--kongdeveloper.netlify.app/gateway/plugins/expressions - new doc for CEL
https://deploy-preview-5449--kongdeveloper.netlify.app/gateway/plugins/expressions-314 - old doc for ATC ‼️ don't review this whole page - it was renamed, so it's actually mostly unchanged. The relevant changes here are the title + warning banner before the intro.
https://deploy-preview-5449--kongdeveloper.netlify.app/gateway/entities/plugin/#conditional-plugin-execution
https://deploy-preview-5449--kongdeveloper.netlify.app/gateway/configure-conditional-plugin-execution/

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[Copilot]

Pull request overview

Updates Kong Gateway conditional plugin documentation for 3.15 by migrating the reference material from legacy ATC expressions to CEL, and adds a version-pinned 3.14 reference page for legacy syntax.

Changes:

• Convert the main plugin-conditions reference (/gateway/plugins/expressions/) from ATC to CEL, including new field/function docs and CEL-focused FAQs.
• Add a 3.14-only legacy reference page (/gateway/plugins/expressions-314/) with a prominent banner pointing readers to the CEL docs for 3.15+.
• Update the conditional plugin execution how-to and the Plugin entity docs to reference CEL-based conditions and the updated logging output.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 8 comments.

File	Description
app/gateway/plugins/expressions.md	Migrates the primary reference page to CEL syntax and expands the field/function reference and FAQs.
app/gateway/plugins/expressions-314.md	Adds a version-scoped 3.14 ATC reference page with a warning banner redirecting 3.15+ readers to CEL docs.
app/_how-tos/gateway/configure-conditional-plugin-execution.md	Updates the how-to TL;DR and logging examples to match CEL-based conditional execution behavior in 3.15.
app/_gateway_entities/plugin.md	Updates the Plugin entity docs’ “Conditional plugin execution” section wording to describe CEL conditions and link to the updated docs.

[chobits]

Although I thought translating from an ERD to a DOC would involve a lot of details, this document surprisingly covers them all and even adds more examples. Awesome! 🤩