[feature]: Copy / View changes as `.diff`

[gameroman]

Describe the feature

A button to view/copy change as a .diff on a /diff pagge

Similar to github's .diff view on a PR, for example:

https://github.com/npmx-dev/npmx.dev/pull/3017 ⬇️
https://github.com/npmx-dev/npmx.dev/pull/3017.diff

Additional information

• Would you be willing to help implement this feature?

Final checks

• Read the contribution guide.
• Check existing issues.