export menu fragments from /docsrc/pages/*.mjs

[jaeh]

abstract:

pages can export menus, those get merged into the state.
this allows automagical menu generation.

questions:

? how will we determine page order

1. using a 'priority' key in the exported menu object.
   no! not easy to change, every page has to be changed without overview over priorities
2. using /docsrc/app.mjs to export an array with menu item priorities.
   no! separates menu order from menu
3. using an array of strings within app.menu to determine order,
   then replace those identifiers with the menu exported from the page.

// /docsrc/pages/page1.mjs
export const state = {
    menu: [
      {
        text: 'page1 link text',
        to: '/page1/', // should be derived automagically
        items: [
          { text: 'subsection link text', to: '#subsection' },
        ],
      },
    ],
}

// /docsrc/pages/page2.mjs
export const state = {
  menu: {
    text: 'page2 link text',
    to: '/page2/', // should be derived automagically
  ],
}

// /docsrc/app.mjs
export const state = {
  menu: [
    { text: 'index page link', to: '/' },  
    ['/page2/', '/page1/', '...'],
  ],
}

[jaeh]

this currently is not possible, state preparation has to change.
first, the state has to be built once for all pages before they get transpiled,