[r12] release notes list wrong platforms and docs link to non-existent extraction help

[joakim-noah]

The release notes for r12 say "Removed all sysroots for platform levels prior to Android 2.3 (API level 10)," but in the linux zip, I see android-9 and then android-12 and up, no android-10. Unless unzip failed me somehow, the platforms need to be changed.

Also, the Setup guide has a link on how to extract the NDK, which goes nowhere now. I know most will know how to unzip it, but given the NDK has used various different extraction methods in the past, I think it's best that it's documented, particularly for the few noobs that will try it.

[DanAlbert]

Thanks for the reports. Sent the CLs yesterday, and it looks like they went live today.