chore: update HowToApplyALicense.md

[ShGKme]

Summary

I'd like to improve HowToApplyALicense.md

1. Update applying to an existing file with a diff
2. Add different files for different file types (add html-like for .vue)
3. Add different samples for different types of source:

The current example doesn't work on frontend. It is blocked by @nextcloud/eslint-config. Which inherits eslint-plugin-jsdoc and checks the license by spdx-expression-parse. So all JS files have AGPL-3.0-or-later instead of recommended by this doc GNU AGPL version 3 or any later version.

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• [ ] Tests (unit, integration, api and/or acceptance) are included
• [ ] Screenshots before/after for front-end changes
• [ ] Documentation (manuals or wiki) has been updated or is not required
• [ ] Backports requested where applicable (ex: critical bugfixes)