docs(operators): write comprehensive docs on delay() and switch()

[staltz]

Description:
I'm starting to document each operator in details and this first PR is just to demonstrate my general approach to document an operator. There are two operators documented here: delay and switch. The structure is:

1. Short headline describing the operator
2. Short "informal" description of the operator. Think of this as "the operator explained for dummies". Rx4 docs suffered from overly-technical and accurate descriptions which made it hard for normal humans to parse in their brains. I'm trying to provide both formal and informal descriptions.
3. Marble diagram
4. Longer formal/technical description
5. Signature: params and return
6. Example code that actually makes sense in the real world
7. Link to the tests

Here is how delay and switch look like. @Blesh @trxcllnt @jhusain your feedback here is important, whether this structure is good. If it's good, I'll just go ahead and do this for all operators.

Related issue (if exists):
#1060, #1242

[benlesh]

Looks great, @staltz.

[frederikschubert]

I am relatively new to reactive programming and if all documentation was like your two examples it would have made my life much easier! 👍
Especially the cross-reference to mergeAll.
I would really like to have the similar operators referenced in the documentation.

[staltz]

Good point @frederikschubert

[benlesh]

This is a point for later, but can we have higher-order observable link to a glossary somewhere that defines this for people? There are a lot of people who don't know what a "higher-order" function is.

Otherwise, LGTM.

[staltz]

Good point, and yes, let's do that.
ESDoc has non-customizable section structure, so we have to choose whether to put a guide on higher-order either in Manual or Tutorial. Anyway, it'll exist.

[coveralls]

Coverage remained the same at 95.982% when pulling a6a6798 on staltz:better-operator-docs into d9855e2 on ReactiveX:master.

[kwonoj]

Merged with 1c4f9f5, thanks @staltz

[lock]

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