docs: Removed reference to a sample notepad app.

The first section incorrectly said that there was a full Notepad
sample app in the "samples" directory; removed that reference.

Also changed the description in "Other Resources and Further Learning"
to point to the sample in "samples/legacy/NotePad".

Doc is staged to: http://asolovay.mtv:9249/training/notepad/

bug: 15612249
Change-Id: I6c17565fd7708fb50915e8cdd73043964cc2878a