Make prev / next links changeable

[MarkusKeck]

Is your feature request related to a problem? Please describe.

Currently it is only possible to customize the text of the links, but not the link itself, this is taken from the sidebar. However, if a page does not exist in the sidebar, no prev / next link can be set or modified.

The current behavior causes the vitepress application to crash if the page is not in the sidebar and you set a prev link via frontmatter, because there is no prev link whose text you can change. As next link always the first page of the sidebar is used.

I think it would be good if you could customize not only the text but also the link.

Describe the solution you'd like

Change the frontmatter options from

---
prev: 'Get Started | Markdown'
---

to

---
prev:
    text: 'Get Started | Markdown'
    link: '/gettingstarted/markdown'
---

Describe alternatives you've considered

No response

Additional context

// prev-next.d.ts
    prev: {
        text: any;
        link: string;
    };
    next: {
        text: any;
        link: string;
    };

Validations

• Follow our Code of Conduct
• Read the docs.
• Read the Contributing Guidelines.
• Check that there isn't already an issue that asks for the same feature to avoid creating a duplicate.

[brc-dd]

PRs are welcome. Need to update this file: https://github.com/vuejs/vitepress/blob/main/src/client/theme-default/composables/prev-next.ts

[MarkusKeck]

@brc-dd Is it mandatory that the application is backward compatible? Should both variants be supported, or is the variant with nested parameters sufficient?

---
prev: Get Started | Markdown
---

---
prev:
    text: Get Started | Markdown
    link: /gettingstarted/markdown
---