[llvm][release] Add links to commonly used release packages (#147719)

This adds download links to the GitHub release pages for common
platforms. The automatically built packages' links are automatically
revealed once the builds are complete. For packages built by hand,
hidden links are included in the text for release uploaders to reveal
later.

The approach taken:
* "LLVM x.y.z Release" becomes the title for this links section.
* Automatically built packages are commented out with special markers so
we can find them to uncomment them later.
* There is placeholder text for the time between release creation and
release tasks finishing.
* Hand built packages have release links but these will need to be
un-commented by release uploaders.
* I have used vendor names for the architectures, that casual users
would recognise.
* Their signature file is linked as well. I expect most will ignore this
but better to show it to remind people it exists.
* I called it "signature" as a generic term to cover the .jsonl and .sig
files. Instructions to use these were added to the text in a previous
change.

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