[BEAM-277] Add transforms section to the programming guide

[melap]

Adds the remaining content for the transforms section: Combine, Flatten, Partition, Side inputs and side outputs

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Jenkins built the site at commit id 512c303 with Jekyll and staged it here. Happy reviewing.

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[asfbot]

Refer to this link for build results (access rights to CI server needed):
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Jenkins built the site at commit id e7940ad 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.

[melap]

R: @davorbonaci

[davorbonaci]

This looks good to me. Merging.

There seem to be issues with rendering both Java and Python SDK links, and not toggling them. However, that doesn't seem to be related to this pull request. So, merging.