[Dynamic Routes]: Add transformPageData configure

[Zhengqbbb]

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

ref: #883 (comment) #2516

When I using Dynamic Routes. To uniformly manage and generate pages with a consistent structure, which involves changing the header title #2516, the suggestion is also to use .vitepress/config.js to use transformPageData

// .vitepress/config.js
{
  // ...
  transformPageData: (pageData, { siteConfig }) => {
    if (page-foo) {
       // change header, title and resolve page data
    } else if (page-bar) {
       // ...
    }
}

Describe the solution you'd like

I think if transformPageData would be in Dynamic Routes configure [pkg].paths.js. Avoid me adding too many logic in .vitepress/config.js, and just managing it in only one place. clear and good management

.
└─ packages
   ├─ [pkg].md         # route template
   └─ [pkg].paths.js   # route paths loader

// packages/[pkg].paths.js
export default {
  paths() {
    return [
      { params: { pkg: 'foo' }},
      { params: { pkg: 'bar' }}
    ]
  },
  transformPageData(pageData) {
    // ...
  }
}

Describe alternatives you've considered

No response

Additional context

No response

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.

[davay42]

Very needed feature. I couldn't figure out how to populate just the 'title:' frontmatter for these dynamic pages without this option. @Zhengqbbb thanks for the tip with use of the transformPageData!