Tweak CEL expression docs

[nscuro]

Clearly separates expression usages for policy conditions from those for notification filters.

Adds a general CEL reference that both contexts can refer to instead of duplicating the same information across pages.

[Copilot]

Pull request overview

This PR reorganizes the documentation for CEL usage by separating policy condition expression details from notification filter expressions, and introducing a shared CEL primer that both can reference.

Changes:

• Adds a new policy-focused reference page for condition expression variables, custom functions, and examples.
• Refactors the general CEL page into a shared syntax/standard-library primer and updates policy/notification docs to link to the right context-specific references.
• Updates reference navigation and cross-links to point to the new vulnerability policies section structure.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
docs/reference/policies/index.md	Updates vulnerability policy reference to point to shared CEL primer + new condition expression page; fixes relative links.
docs/reference/policies/condition-expressions.md	New comprehensive reference for policy condition contexts, variables, functions, and examples.
docs/reference/policies/.pages	Adds section nav for vulnerability policies and condition expressions.
docs/reference/notifications/filter-expressions.md	Updates CEL references to the shared primer and clarifies function availability for notification filters.
docs/reference/index.md	Reorganizes reference landing page sections and adds links to the new policy/condition expression pages.
docs/reference/cel-expressions.md	Converts CEL page into a shared syntax/standard library primer with context-specific pointers.
docs/reference/.pages	Updates vulnerability policies nav entry to point to the new policies/ section directory.
docs/guides/user/managing-vulnerability-policies.md	Updates reference links to the new vulnerability policies reference location.
docs/concepts/vulnerability-policies.md	Updates reference links to the new vulnerability policies reference location.

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