[FLINK-5258] [docs] reorganize the docs to improve navigation and reduce duplication

[alpinegizmo]

Goals:

• improve navigation and discoverability: important topics shouldn't be hard to find
• reduce duplication: ideally every piece of information would have one natural place to be
• arrange the content in a natural, linear ordering for those who want to read (or skim through) everything

This pull request moves content around, and in some cases merges sections with major overlap. When I noticed them I fixed small wording problems and typos, and I did somewhat more significant editing in the concepts section.

The results can be reviewed at https://alpinegizmo.github.io/flink-docs/

[StephanEwen]

Thanks a lot for this pull request!
I think this looks very good - much better than the current state.

Two changes I would make:

• Drop "Projects and Dependencies", it is so outdated that it is misleading these days
• The same holds for "How to add a new DataSet Operator"

Otherwise, I would merge this, as it is a big improvement over the current version.

[StephanEwen]

Thanks for addressing this.
Looks good to me overall, merging this...

[rohitagarwal003]

We seem to have lost changes from c024b0b and 91414d9 during the reorg. :-(

I wonder if we lost more such changes.

cc - @alpinegizmo @fhueske @mxm

[fhueske]

Hi @mindprince, thanks a lot for pointing out this issue.

I had a look at the diffs and found that the following changes got lost or refactored:

• c024b0b : ./docs//setup/fault-tolerance.md and ./docs/dev/api_concepts.md
• 91414d9 : ./docs//setup/fault-tolerance.md
• 3698379: ./docs//setup/fault-tolerance.md

Since the docs have been refactored, we should check where these changes need to go.