add notify, files and utils modules documentation

samples/files_sample.lua

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+function on_resume()
+    files:delete("abc.txt")
+    files:write_text("abc.txt", "This is text")
+    local string = files:read_text("abc.txt")
+    ui:show_text(string)
+end

samples/notify_sample.lua

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+
+local curr_notab = {}
+local curr_titletab = {}
+local curr_keystab = {}
+
+function on_resume()
+    notify:get_current()
+end
+
+function on_notify_posted(n)
+    curr_notab[n.key] = n
+    redraw()
+end
+
+function on_notify_removed(n)
+    curr_notab[n.key] = nil
+    redraw()
+    ui:show_toast("Notify from "..n.package.." removed")
+end
+
+function redraw()
+    fill_tabs(curr_notab)
+    ui:show_lines(curr_titletab)
+end
+
+function on_click(i)
+    notify:open(curr_keystab[i])
+end
+
+function fill_tabs(tab)
+    curr_titletab = {}
+    curr_keystab = {}
+
+    for k,v in pairs(tab) do
+        table.insert(curr_titletab, v.title)
+        table.insert(curr_keystab, v.key)
+    end
+end