FIX - updated readme file to provide a functioning demo setup

[Agnul97]

The docker-compose deployment file, in the absence of local docker images, tries to pull the consumers from the docker hub which are, for now, not present in the Kapua docker repo. This means that deploying Kapua following the "demo setup" section of the main project readme file is actually not possible.

In order to provide a functioning demo setup, I changed the instructions provided for that section, explaining that it is possible to exploit the hub in order to deploy tagged released versions but not for snapshot versions (for example, the actual development branch with the consumers), which require a local build.
Furthermore, I updated the building section of the documentation, because I found some improvements there. I attached this update in this PR because the "quick demo" section actually depends on that document.

[codecov]

Codecov Report

Merging #3777 (dd8bc69) into develop (fd877d7) will increase coverage by 3.00%.
The diff coverage is 32.50%.

❗ Current head dd8bc69 differs from pull request most recent head f10519b. Consider uploading reports for the commit f10519b to get more accurate results

@@              Coverage Diff              @@
##             develop    #3777      +/-   ##
=============================================
+ Coverage      49.19%   52.19%   +3.00%     
  Complexity        26       26              
=============================================
  Files           1865     1866       +1     
  Lines          35275    35284       +9     
  Branches        2786     2782       -4     
=============================================
+ Hits           17352    18416    +1064     
+ Misses         17003    15895    -1108     
- Partials         920      973      +53     

Impacted Files	Coverage Δ
...pse/kapua/job/engine/commons/logger/JobLogger.java	0.00% <0.00%> (ø)
...resources/v1/resources/UserCredentialFiltered.java	0.00% <0.00%> (ø)
...ice/endpoint/internal/EndpointInfoServiceImpl.java	82.25% <86.66%> (+1.96%)	⬆️

... and 137 files with indirect coverage changes

[Coduz]

After further review also of the linked issue I think that this solution won't work.
Checking out release-1.7.x would have the same problem, since snapshot images are not hosted in Docker HUB.

The best solution would be to specify that only released version have Docker Images hosted in Docker HUB. For develop version we need to specify that the user needs to build the images locally if he wants to run a demo on develop. Otherwise it needs to checkout a tagged release.