Add minimal docs for Asset class (to fix broken link to same)

content/docs/api/db/asset/children/contents.lr

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+title: children
+---
+summary: Returns a query to the children of a page.
+---
+type: property
+---
+body:
+
+[Assets :ref](../) that correspond to directories can have children
+below them.  The `children` attribute provides a convenient way to access
+those. It returns an iterable of child assets.

content/docs/api/db/asset/contents.lr

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+title: Asset
+---
+summary: Static assets, copied verbatim to the output.
+---
+module: lektor.assets
+---
+type: class
+---
+body:
+
+Assets are [Source Objects :ref](../obj/) that correspond to static files that
+are copied verbatim from the `/assets` folder to the output directory.
+
+Usually, there is no need to manipulate Asset objects directly.

content/docs/api/db/asset/get-child/contents.lr

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+title: get_child
+---
+body:
+
+Lookup a child of the asset.  Returns `None` if no child exists with the specified name.
+
+Naturally, this method always returns `None` unless the asset is a directory.
+---
+signature: name, from_url=False
+---
+summary: Get child asset
+---
+type: method