[BEAM-9682] Move existing Go SDK katas fixed time window lesson into windowing section

[damondouglas]

This pull request moves an existing lesson on fixed time windows into a new section called windowing. As we discussed, no content has been changed and therefore the stepik course has already been updated.

I would like to request the following reviewer:

(R: @lostluck )

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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	Whitespace	Typescript
Non-portable
Portable	---		---	---	---	---

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GitHub Actions Tests Status (on master branch)

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[lostluck]

LGTM Thanks!