[BEAM-7390] Add combinevalues code snippets

[davidcavazos]

Add CombineValues code snippets.

R: @aaltay
R: @rosetn

Staged: http://apache-beam-website-pull-requests.storage.googleapis.com/12328/documentation/transforms/python/aggregation/combinevalues/index.html

---

Thank you for your contribution! Follow this checklist to help us incorporate your contribution quickly and easily:

• Choose reviewer(s) and mention them in a comment (R: @username).
• Format the pull request title like [BEAM-XXX] Fixes bug in ApproximateQuantiles, where you replace BEAM-XXX with the appropriate JIRA issue, if applicable. This will automatically link the pull request to the issue.
• Update CHANGES.md with noteworthy changes.
• If this contribution is large, please file an Apache Individual Contributor License Agreement.

See the Contributor Guide for more tips on how to make review process smoother.

Post-Commit Tests Status (on master branch)

Lang	SDK	Dataflow	Flink	Samza	Spark	Twister2
Go		---		---		---
Java
Python				---		---
XLang		---		---		---

Pre-Commit Tests Status (on master branch)

---	Java	Python	Go	Website
Non-portable
Portable	---		---	---

See .test-infra/jenkins/README for trigger phrase, status and link of all Jenkins jobs.

[aaltay]

LGTM. I can merge after @rosetn gets a chance to review. Please ping me after that.

[rosetn]

I think you might have embedded the wrong code sample for example 1--you're using saturated_sum instead of just sum and the output is the result of sum

[davidcavazos]

I think you might have embedded the wrong code sample for example 1--you're using saturated_sum instead of just sum and the output is the result of sum

Thank you for catching this!

[aaltay]

Is this ready to be merged now? Seems like the snippet issue is fixed.

[rosetn]

LGTM @aaltay ready to merge.