Page don't and won't use dot references as in `page1.page2`.

Use

```
children: page1 page2 page3
```

in the frontmatter

Children order specifies order in an index page for the current directory. It
has to specify what is a page and what is a directory, as we can have:

```
doc/
  index
  foo
  foo/
    index
    bar
```

In the specification of the order in `doc/index`, we must make the difference
between the foo page and the foo directory. This was not practical with
references. So instead, the order specification has its own type.

Allow sequences of spaces in a row: `children:      page1           page2`.

Unexpose unnecessary functions
open modules
use mli for model/frontmatter

Co-authored-by: Jules Aguillon <[email protected]>

Directly take children order into account.

Co-authored-by: Jules Aguillon <[email protected]>

Co-authored-by: Jules Aguillon <[email protected]>

Co-authored-by: Jules Aguillon <[email protected]>

But alias it to Id. It was bringing too many values in scope (eg shadowing
compare).