Static file templates have no Date function available

[Jc-L]

Version:

• listmonk: 2.5.1
• OS: Debian 11

Description of the bug and steps to reproduce:
Some function, like {{ Date "2006" }}, throw an error when used in static file public/templates/index.html. Use case was to have dynamic (c) date in the footer.

Logs:
2023/09/15 07:39:53 init.go:758: error parsing public templates: template: message.html:43: function "Date" not defined

Expected behaviour:
Make it possible to use function described in templating documentation, possibly also the Sprig function. If not, clarify limitation in documentation.