Convert Marbles to SVG and rework diagrams

[simonbasle]

Source for choice of fonts: http://www.ars-informatica.ca/article.php?article=59

Changes:

[doc] Rework marble diagrams into SVG
This commit is a large rework of the marble diagrams:

• started from the omnigraffle original exported to SVG
• large rework of diagrams with clearer and more consistent semantics
• large naming rework to be more consistent with operator names
• creation of missing marble diagrams
• javadoc modifications to point to new diagram names
• main javadocs no longer point to URLs but expect an embedded
  doc-files set up
• SVG use Tahoma/Nimbus and Lucida Console/Courier New font families

[doc] Add "Contributing to SVG" README and task to copy SVG to doc-files

Said task is run as part of processResources, which gives a chance that new
users checking out the repo will see the quick documentation with diagrams
after first building the project.

[simonbasle]

The diagrams are from #1377 by @thekalinga but the font family have been changed to not require any font download (and to be able to embed the SVG directly as is in the javadoc jar without mandatory text-to-path+optimization steps nor double committing of raw/optimized SVGs). Also the javadoc src changes have been reused from this PR with slight modifications and limiting the diff mostly to <img> rows.

[simonbasle]

NB: now that the overall set up is finalized, we will need to validate the content of the diagrams themselves one by one vs the current release javadoc. Best way to do that is to produce a snapshot of the javadoc and visually compare it to the current released javadoc.

[codecov-io]

Codecov Report

Merging #1390 into master will increase coverage by 0.13%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master    #1390      +/-   ##
============================================
+ Coverage     84.11%   84.24%   +0.13%     
+ Complexity     3897     3895       -2     
============================================
  Files           359      358       -1     
  Lines         29789    29419     -370     
  Branches       5522     5463      -59     
============================================
- Hits          25057    24785     -272     
+ Misses         3096     3034      -62     
+ Partials       1636     1600      -36

Impacted Files	Coverage Δ	Complexity Δ
.../java/reactor/core/publisher/BlockingIterable.java	71.87% <0%> (-6.25%)	7% <0%> (ø)
.../java/reactor/core/publisher/FluxContextStart.java	69.81% <0%> (-3.78%)	2% <0%> (ø)
.../reactor/core/publisher/FluxDelaySubscription.java	75.86% <0%> (-3.45%)	3% <0%> (ø)
...re/src/main/java/reactor/util/context/Context.java	82.6% <0%> (-2.01%)	17% <0%> (-2%)
...or-core/src/main/java/reactor/core/Exceptions.java	90.51% <0%> (-1.73%)	49% <0%> (-1%)
.../java/reactor/core/scheduler/ElasticScheduler.java	83.47% <0%> (-1.66%)	26% <0%> (ø)
...re/src/main/java/reactor/core/publisher/Hooks.java	95.23% <0%> (-1.09%)	35% <0%> (-6%)
...in/java/reactor/core/publisher/FluxOnAssembly.java	90.9% <0%> (-0.86%)	20% <0%> (+4%)
...rc/main/java/reactor/core/publisher/FluxRange.java	84.76% <0%> (-0.67%)	6% <0%> (ø)
...ain/java/reactor/core/publisher/FluxConcatMap.java	89.85% <0%> (-0.57%)	7% <0%> (ø)
... and 27 more

---

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[simonbasle]

@smaldini @thekalinga in the last commit I decided to put the SVG directly in the doc-files folder within the repo. That way, switching between master and 3.1.x doesn't show a ton on uncommitted files and there is no set up step to let the IDE correctly display diagrams from local filesystem.

The root folder can still later contain reference-specific diagrams and we can always shuffle these around a bit (but for now they are referenced by URL anyway)

[thekalinga]

@simonbasle This is not an issue per se. But thought it would help if you too know this. In some marble diagrams (create & push as far as I remember), svgs have java code that uses Fira Code font that makes the code look much readable

This is the only place where I used font other than IBM Plex Sans. So incase you have not updated there aswell, please do so

[simonbasle]

@thekalinga thanks for letting me know. Yeah I saw the Fira Code and replaced with "web safe fonts" Lucida Console (available by default on windows OS) with fallback to Courier New (available by default on most if not all OS)

[simonbasle]

Rendering of marbles inside jar tested in Eclipse (might be subpar, some arrowheads look weird, but let's see if people complain) and IntelliJ IDEA 2018.3.1 (rendering size bug has been fixed in this version)

[bsideup]

Looks really good in general! Just have a couple of things:

• Flux#concatMapDelayError uses concatMap diagram
• Mono.fromCompletionStage & Mono.fromFuture(Supplier) contains diagram of fromFuture (with .get() call, althought it uses whenComplete instead)

[simonbasle]

@bsideup so far no Flux combining operator has a *DelayError variant of the diagram. I don't think it is super critical, but if it comes up a couple times more we can always add them.
The get() is from the Supplier. I added Supplier#get() to make it clearer.

[simonbasle]

Between xmas and new year, 'tis time to merge this PR 🎅