[FLINK-12639] [docs] add "getting started" section to documentation

[knaufk]

What is the purpose of the change

Add a "Getting Started" section to the documentation to expand in the course of FLIP-42. This change only the existing content into the target structure.

Brief change log

see above

Verifying this change

build docs and check result

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-reord code paths (performance sensitive): no
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Yarn/Mesos, ZooKeeper: no
• The S3 file system connector: no

Documentation

• Does this pull request introduce a new feature? no
• If yes, how is the feature documented? not applicable

[flinkbot]

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Review Progress

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

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[sjwiesman]

@knaufk built locally and looks good +1, feel free to have someone merge

[knaufk]

@sjwiesman Thanks a lot for review.

[NicoK]

@flinkbot approve all