docs: add signing identity examples and link to how-to guides

[kshivani06]

I have updated the documentation for --identity in the package reference guide to provide better clarity on valid inputs for different platforms.

• Indicative Examples: Added examples for macOS (40-character SHA-1 hex strings/certificate names) and Windows (thumbprints/subject names) to show the "shape" of valid values.
• Focused Content: Maintained a concise reference guide by linking directly to the platform-specific how-to guides for detailed signing instructions rather than duplicating them.
• Path Verification: Verified that relative links correctly point to docs/en/how-to/code-signing/
• Scope Management: Omitted any mention of distribution complexities or CI/GitHub Actions to keep the reference guide strictly focused on tool usage.

PR Checklist:

• All new features have been tested
• All new features have been documented
• I have read the CONTRIBUTING.md file
• I will abide by the code of conduct

[freakboy3742]

Thanks - I've made some tweaks to your original draft, but this was a really helpful starting point.

For future reference, see the notes that I've left on the changenote file.

[freakboy3742]

A few details here for future reference:

• A changenote file should not be empty
• This should have a .md extension
• In this case, it's probably a misc, not a doc. Yes, it's changing documentation; but the purpose of changenote files is to be a first draft of the release notes. A relatively minor (however valuable) addition to docs isn't something that really needs to be in the release notes of the next version.

[kshivani06]

Thanks for the feedback and for the final tweaks! I've noted the point about using .misc for minor doc updates instead of .doc for future reference.