Merge release-plan and download pages

Create this ticket according to the discussion in [ r48518| https://reviews.apache.org/r/48518/ ]

From janisz
It duplicates data from Downloads, maybe it could be merged into one page.

From jieyu
This is slightly different from Downloads, because it might also contains information about a future release (e.g., what will be the major feature expected for a future release).

I agree with you that there's some duplication right now. Let's create a ticket to track the merge of these two pages. Once that's resolved, we can use the same link from the home page? An alternative is have a link in the download page to point to this page. But I like the idea of merging the Download page with this!