docs(aio): Added developer guide on RxJS

[brandonroberts]

Please check if the PR fulfills these requirements

• The commit message follows our guidelines: https://github.com/angular/angular/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

What kind of change does this PR introduce? (check one with "x")

[ ] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[x] Other... Please describe:

Developer guide on observable usage. This was migrated over from angular/angular.io#3047

What is the current behavior? (You can also link to an open issue here)

What is the new behavior?

Does this PR introduce a breaking change? (check one with "x")

[ ] Yes
[ ] No

If this PR contains a breaking change, please describe the impact and migration path for existing applications: ...

Other information:

[mary-poppins]

The angular.io preview for 49b1b705870d35e2c53755d71641e5772643f6bb is available here.

[mary-poppins]

The angular.io preview for 7d154136e25bee0b4f2e29a30788a7136d83d0d2 is available here.

[mary-poppins]

The angular.io preview for 1afc491617a7c397e692de72997403f36651472a is available here.

[mary-poppins]

The angular.io preview for a481925ced0ecc78cb3a7a425e89b1acf01b240b is available here.

[gkalpak]

Is it intentional that pipes are added here, while they are already present above (around line 188)?

[brandonroberts]

I doubt it. Probably happened during a rebase.

[Foxandxss]

Where is the plunker? Rxjs is a complex subject and there is a lot of examples that you don't know what kind of output to expect.

Continuing on that, there are a few simple snippets that could use some comments saying what is the expected output.

I saw you named hot and cold but there is no more on it. A link would be nice.

There is a part where you give a few resources link. That is better to be at the end. To not distract people from reading your guide.

I find the first part (Observable is a function) to be too complicated.

[Foxandxss]

I am not sure about this sentence (disclaimer, I am a bit noobish with rxjs). Observables can be used for that, but they are much more.

Would make sense to say ...

Observables are a programming technique that can be used for handling asynchronous and event-based values procuded over time

And even so, I am not so sure :/

[Foxandxss]

Why a link to benlesh fork and not the real deal?

[Foxandxss]

I don't understand the first sentence.

[Foxandxss]

Why a label for ID here?

[mary-poppins]

You can preview 7fa4b70 at https://pr16452-7fa4b70.ngbuilds.io/.

[mary-poppins]

You can preview 84c07c9 at https://pr16452-84c07c9.ngbuilds.io/.

[mary-poppins]

You can preview 9012622 at https://pr16452-9012622.ngbuilds.io/.

[mary-poppins]

You can preview 0bb8174 at https://pr16452-0bb8174.ngbuilds.io/.

[mary-poppins]

You can preview a705287 at https://pr16452-a705287.ngbuilds.io/.

[mary-poppins]

You can preview 4c46d6c at https://pr16452-4c46d6c.ngbuilds.io/.

[mary-poppins]

You can preview a4a1f76 at https://pr16452-a4a1f76.ngbuilds.io/.

[mary-poppins]

You can preview f840570 at https://pr16452-f840570.ngbuilds.io/.

[mary-poppins]

You can preview 7ab406e at https://pr16452-7ab406e.ngbuilds.io/.

[mary-poppins]

You can preview bd0bad4 at https://pr16452-bd0bad4.ngbuilds.io/.

[jenniferfell]

@jasonaden : Recommendation here? Has all of this been incorporated into the new doc you're working on, so this can be closed? Other next steps on this PR? Thanks.

[jenniferfell]

@jasonaden Can we close this as a duplicate of #21423 ?

[splincode]

@brandonroberts what is the status of your pull request? is it still relevant? please fix conflicts if they exist

[brandonroberts]

@splincode this guide has been folded into the Observables guide except for the marble testing area, which will come along later. I think this can be closed.

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