Add reference documentation for secret()

[Gijsreyn]

PR Summary

This pull request adds the reference documentation for the secret() function. Also, minor tweaks were made to the CLI command function to accommodate the addition of more functions without breaking the documentation.

PR Context

[Gijsreyn]

@michaeltlombardi After revisiting the examples, I have removed examples 3 and 4. Additionally, examples 1 and 2 have been rewritten, as they cannot be used in a string concatenation (still wondering why it couldn't be done even though the ARM docs also always directly use it on a property).

[Copilot]

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

Adds reference documentation for the secret() function and adjusts CLI function-list docs formatting to support additional functions going forward.

Changes:

• Added a new reference page describing secret() syntax, parameters, examples, and security considerations.
• Modified the CLI function index page content/formatting (including a switch to jsonc fencing and truncation of a function list snippet).

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
docs/reference/schemas/config/functions/secret.md	New reference page documenting the secret() configuration document function.
docs/reference/cli/function/index.md	Updates the CLI docs for listing functions (but currently replaces a section with a truncation placeholder).

[michaeltlombardi]

@SteveL-MSFT - LGTM, could you give a quick final check for technical accuracy?