docs/man: minor cleanup

- Move lua filter to separate file
- Use a subshell to write `nixvim-header.5`
- Use bash function to avoid repetition

docs/man/filter.lua

@@ -0,0 +1,15 @@
+local text = pandoc.text
+
+function Header(el)
+	if el.level == 1 then
+		return el:walk({
+			Str = function(el)
+				return pandoc.Str(text.upper(el.text))
+			end,
+		})
+	end
+end
+
+function Link(el)
+	return el.content
+end