Support custom `$nav$` rendering using templates

`$nav$` currently generates hard-coded HTML (see https://github.com/jgm/yst/blob/e3838d318318012d9237c9a34e792d1a630805d5/Yst/Render.hs#L77). It would be great if users could specify a template for it instead.

If you're interested I can work on this and file a PR.

## Possible solution

Custom nav templates could be specified by setting an optional setting in the config.

#### config.yaml

```
navigation: navmenu.st
```

If it isn't set `$nav$` in a layout template could fallback to a default template which renders the same HTML yst does now. The default would be something like this:

#### navmenu.st

```

$if(it.page)$

• 
  $it.title$
  

$else$
$it.title$
$it.entries:navmenu()$
$endif$

Start in Yst/Render.hs at the $nav$ rendering code linked in the issue, then inspect how config.yaml settings and templates are loaded. Implement the optional navigation setting and preserve the existing HTML through a default template when it is absent. Done means custom navmenu.st rendering works and the fallback matches current output.

Written by the indexing model from the issue text.