[FLINK-16210][docs] Extending the "Flink Architecture" section.

[morsapaes]

What is the purpose of the change

This PR extends the "Flink Architecture" section of the docs with more information about Flink Master components and application execution (incl. the new Application Cluster mode).

Brief change log

• Modified flink-architecture.md

Verifying this change

This change is a trivial rework / code cleanup without any test coverage.

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

• Does this pull request introduce a new feature? no

[morsapaes]

Pinging @aljoscha and @alpinegizmo (who had this rework in mind as well).

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Automated Checks

Last check on commit 41c4429 (Fri Oct 16 10:54:29 UTC 2020)

Warnings:

• Documentation files were touched, but no .zh.md files: Update Chinese documentation or file Jira ticket.

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Review Progress

• ❓ 1. The [description] looks good.
• ❓ 2. There is [consensus] that the contribution should go into to Flink.
• ❓ 3. Needs [attention] from.
• ❓ 4. The change fits into the overall [architecture].
• ❓ 5. Overall code [quality] is good.

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[aljoscha]

Thanks a lot for working on this! I pushed two commits with suggestions.

Another (slightly more complicated) thing is that the rest of the documentation changed and some of the links in this are now outdated. If you could rebase this and change the links that use {{ site.baseurl }} ... to use this syntax {% link dev/stream/state/checkpointing.md %} building the docs will fail on missing links.

[alpinegizmo]

I'm pretty happy with the page as a whole. It covers a coherent set of topics in a helpful way, and connects to the other parts of the documentation that need to be understood alongside this.

Here and there I found some small grammatical issues. And there are a few points where I think other changes are needed. I've provided suggestions in most cases.

[morsapaes]

Thanks for reviewing, @aljoscha and @alpinegizmo! It's weird that the links worked fine in the local build even with the duplicated reference to the base URL — I've corrected that and will also update the Documentation Style Guide.

[aljoscha]

Thanks again, @morsapaes! I merged this.