internal/godocfx: better styling of package doc

[tbpg]

Package docs have a few issues:

• No syntax highlighting for code blocks. We need class="prettyprint".
• Markdown style links don't work.
• Markdown style lists don't work.

These are all issues with the underlying doc.ToHTML that we use. We can't switch to a Markdown parser directly for the package doc because there are the magic GoDoc headers that don't start with #.

google-cloud-go/internal/godocfx/parse.go

Lines 576 to 580 in f321666

	func toHTML(s string) string {
	buf := &bytes.Buffer{}
	doc.ToHTML(buf, s, nil)
	return buf.String()
	}

[tbpg]

Initial Markdown support in #3816.