[FLINK-18377] Rename "Flink Master" back to JobManager in documentation

[aljoscha]

What is the purpose of the change

Please see https://issues.apache.org/jira/browse/FLINK-18377 for a full description. The gist of it is that we want to get rid of the word "master".

Brief change log

• change glossary.md/glossary.zh.md to update definition of JobManager and JobMaster
• rename mem_setup_master.md to mem_setup_jobmanager.md (same for .zh). I didn't create redirects for this because the file was newly added for Flink 1.11
• change occurences of Flink Master to JobManager in the rest of the docs
• on the way I also changed a couple of links to use the better {% link ... syntax, this should help catch dead links better
• add a second commit to update Jekyll from 4.0.0 to 4.0.1 to fix compile warnings on Ruby 2.7

There is still an entry for JobMaster in the glossary. I could remove that one as well. The only mentions of master in the doc are now for the JobMaster.

Verifying this change

• manually check renamings
• build documentation to see that all links are still valid

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): no
• The public API, i.e., is any changed class annotated with @Public(Evolving): no
• The serializers: no
• The runtime per-record code paths (performance sensitive): no
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn/Mesos, ZooKeeper: no
• The S3 file system connector: no

Documentation

It's a pure documentation change.

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Last check on commit e090f55 (Fri Jun 19 12:50:40 UTC 2020)

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[zentol]

Maybe do another sweep for "Masters"

[aljoscha]

I pushed changes, PTAL.

Also, thanks for the speedy review!

[aljoscha]

For the latest commits:
https://ci.apache.org/builders/flink-docs-docs_experimental__docs/builds/4

[zentol]

+1

[aljoscha]

Thanks for the reviews! I merged.

Btw, I thought about adding release notes but we didn't announce the earlier renaming and the documentation was in enough of a mixed state that I would do this silently.