Add safeURL and urldecode template functions

[CommitToday]

This PR addresses issue #4710 by adding two new template functions to improve URL handling in Alertmanager notifications:

1. safeURL function: An alias for the existing safeUrl function to provide consistency with common naming conventions. Both functions now work identically.

2. urldecode function: A new function that URL-decodes strings. This is particularly useful for decoding query parameters or other URL-encoded data in notification templates. The function returns the original string if decoding fails, ensuring template rendering doesn't break on invalid encoding.

The changes include:

• Implementation of both functions in template/template.go
• Documentation updates in docs/notifications.md
• Comprehensive tests covering normal usage, complex encoding, and error cases

These additions enhance the flexibility of notification templates when working with URL-encoded data.