Updated tutorials

[jmarcosfer]

• Fixed/deleted some broken links on "1. Getting started.md"
• Created new tutorial for real-time usage, moved parts that were previously in "1. Getting started.md"
• Created new tutorial for DL inference with pretrained models ("3. Running DL models with Tensorflow.js")
  • this is perhaps too long? it could be broken down into two: deferred time and real-time
• "4. Developer notes.md" still mostly empty: a lot of the recommendations are already in "2. Real-time analysis.md" and "3. Running DL models with Tensorflow.js"

[albincorreya]

@jmarcosfer, In addition, it will be nice to have an index for all of our pre-trained ML models somewhere in the documentation like how it is listed here in tfjs-models repo. So, that the devs get the correct hyperlinks to the various models distributed on our model repo website. It seems not very user-friendly to find tfjs specific models from this pool of models in https://essentia.upf.edu/models/. cc @palonso @dbogdanov What do you all think?