[BEAM-2142] Add composite transforms Python snippets to programming guide

[melap]

Add new snippets from BEAM-1926
R: @aaltay

[asfbot]

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Jenkins built the site at commit id 1061b82 with Jekyll and staged it here. Happy reviewing.

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[melap]

Have PR #228 out to fix the broken link (unrelated to this change)

[melap]

retest this please

[asfbot]

Refer to this link for build results (access rights to CI server needed):
https://builds.apache.org/job/beam_PreCommit_Website_Stage/445/

Jenkins built the site at commit id 1061b82 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.

[aaltay]

LGTM