[BEAM-2869] Add dates to releases on downloads page

[melap]

resolves #312

[asfgit]

Refer to this link for build results (access rights to CI server needed):
https://builds.apache.org/job/beam_PreCommit_Website_Stage/684/

Jenkins built the site at commit id 620e761 with Jekyll and staged it here. Happy reviewing.

Note that any previous site has been deleted. This staged site will be automatically deleted after its TTL expires. Push any commit to the pull request branch or re-trigger the build to get it staged again.

[lukecwik]

LGTM

[lukecwik]

@asfgit merge

[asfgit]

MergeBot: Merge: PR 318 in state error with description Unexpected Error: 502 Server Error: Proxy Error for url: https://builds.apache.org/job/beam_PreCommit_Website_Merge/43/api/python?depth=1&tree=building.. URL: http://mergebot-vm.apache.org/beam-website/318.

[asfgit]

Error: MergeBot encountered an unexpected error while processing this PR: 502 Server Error: Proxy Error for url: https://builds.apache.org/job/beam_PreCommit_Website_Merge/43/api/python?depth=1&tree=building.

[tgroh]

@asfgit merge