Add doc.go descriptions to packages

pkg/asset/doc.go

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+// Package asset provides an embedded filesystem that serves client-side CSS and JS.
+package asset

pkg/document/doc.go

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+// Package document implements Markdown document processing and serving.
+package document

pkg/source/doc.go

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+// Package source contains various file sources for Markdown documents.
+package source

pkg/sse/doc.go

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+// Package sse implements server-sent events as described by the HTML Standard.
+// See https://html.spec.whatwg.org/multipage/server-sent-events.html for more details.
+//
+// Currently, only a server implementation is provided.
+package sse