Adds docs for pub/sub related classes and methods

[mushketyk]

What does this PR do?

Adds documentation for new pub/sub functionality in web3j implemented in #458

Where should the reviewer start?

Just go through new documentation in getting_started.rst

Why is it needed?

To allow users to quickly start working with pub/sub functionality in web3j.

[codecov]

Codecov Report

Merging #768 into master will decrease coverage by 0.14%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #768      +/-   ##
============================================
- Coverage     77.38%   77.23%   -0.15%     
+ Complexity     1851     1848       -3     
============================================
  Files           241      241              
  Lines          6814     6814              
  Branches       1012     1012              
============================================
- Hits           5273     5263      -10     
- Misses         1291     1300       +9     
- Partials        250      251       +1

Impacted Files	Coverage Δ	Complexity Δ
...n/java/org/web3j/protocol/core/filters/Filter.java	53.12% <0%> (-14.07%)	8% <0%> (-2%)
core/src/main/java/org/web3j/utils/Flowables.java	80% <0%> (-4%)	9% <0%> (-1%)
...org/web3j/protocol/websocket/WebSocketService.java	85.26% <0%> (-1.06%)	50% <0%> (-1%)
.../main/java/org/web3j/protocol/rx/JsonRpc2_0Rx.java	74.07% <0%> (+2.46%)	26% <0%> (+1%)	⬆️

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[snazha-blkio]

Hi @mushketyk

Thanks for your hard work, we created a branch off your PR and some minor tweaks, given that we couldn't make changes to your PR.

This has been merged and can be found here: #791

Thanks for your contribution!