Improve documentation about reviewing add-ons from the store (#15887)
Part of issue #15576.
Summary of the issue:
Now, add-ons can be reviewed from GitHub discussions, linked in the Available add-ons tab of the store.
Description of user facing changes
This improves documentation for this feature:
Explained that this action can be found in the Available add-ons tab.
Remove reference to comment on addon versions, since comments aren't longer added to add-ons metadata.