[BEAM-4498] Sync javadoc and pydoc from apache/beam-site

[udim]

Migrates generated SDK documentation over to apache/beam on the asf-site branch.

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Post-Commit Tests Status (on master branch)

Lang	SDK	Apex	Dataflow	Flink	Gearpump	Samza	Spark
Go		---	---	---	---	---	---
Java
Python		---		---	---	---	---

[udim]

R: @swegner

The flow as I see it:

• Release script generates 2 PRs:
  • The first one generates pydoc and javadoc dirs (like this one), and is to be merged to branch asf-site.
  • The second updates website/documentation/sdks/javadoc/{index,current}.md, and is to be merge to branch master.
• Post-commit process generates new files from the second PR and puts them in asf-site.
• Apache website publishing process picks up changes from asf-site branch.

[tweise]

@swegner @udim are we not considering an alternative process that does not involve pulling all this into the repository?

[robertwb]

Please don't check in 4 million lines of auto-generated docs into the main repository. (Is there a doc explaining what our plan is?)

[tweise]

I think this requires more discussion before moving ahead.

[udim]

@swegner updated the doc yesterday with this section: https://docs.google.com/document/d/1lfbMhdIyDzIaBTgc9OUByhSwR94kfOzS_ozwKWTVl5U/edit#heading=h.uqzivheohd7j

[swegner]

@robertwb @tweise Let me restart the conversation on the dev@ list; this is a step in the implementation of the Beam-Site Automation Reliability work discussed here: https://s.apache.org/beam-site-automation.

As @udim mentioned we added a section to the doc for Concerns about generated content in apache/beam repository.

[udim]

Abandoning, per discussion.