docs: Updating N download pages for DP4

Verified the checksums against the image files before uploading
them to cloud share. For OTA versions, only the SHA256 checksums were
provided; I verified those checksums against the original image files,
then generated the MD5 and SHA1 checksums myself.

Once all files were staged to cloudshare, I re-downloaded them and
verified them against the checksums I'd added to the preview HTML
page; everything checks out.

See first comment for doc stage location.

bug: 28981896
Change-Id: I10165d402c4f7f07cad267f460b289a7bbf6b858