[docs] rewriting the tutorial.

docs/guide/introduction.rst

@@ -1,8 +1,8 @@
 Introduction
 ============
 
-The first thing you need to understand before you use Bonobo, or not, is what it does and what it does not, so you can
-understand if it could be a good fit for your use cases.
+The first thing you need to understand before you use |bonobo|, or not, is what it does and what it does not, so you
+can understand if it could be a good fit for your use cases.
 
 How it works?
 :::::::::::::
@@ -13,7 +13,10 @@ terminals and source code files.
 It is a **data streaming** solution, that treat datasets as ordered collections of independant rows, allowing to process
 them "first in, first out" using a set of transformations organized together in a directed graph.
 
-Let's take a few examples:
+Let's take a few examples.
+
+Simplest linear graph
+---------------------
 
 .. graphviz::
 
@@ -26,18 +29,35 @@ Let's take a few examples:
         BEGIN -> "A" -> "B" -> "C" -> "END";
     }
 
-One of the simplest, by the book, cases, is an extractor sending to a transformation, itself sending to a loader.
+One of the simplest, by the book, cases, is an extractor sending to a transformation, itself sending to a loader (hence
+the "Extract Transform Load" name).
+
+.. note::
+
+    Of course, |bonobo| is aiming at real-world data transformations and can help you build all kinds of data-flows.
+
+Bonobo will send an "impulsion" to all transformations linked to the `BEGIN` node (shown as a little black dot on the left).
+
+On our example, the only node having its input linked to `BEGIN` is `A`.
 
-Bonobo will send an "impulsion" to all transformations linked to the little black dot on the left, here `A`.
 `A`'s main topic will be to extract data from somewhere (a file, an endpoint, a database...) and generate some output.
-As soon as the first row of `A`'s output is available, Bonobo will start asking `B` to process it. As soon as the first
-row of `B`'s output is available, Bonobo will start asking `C` to process it.
+As soon as the first row of `A`'s output is available, |bonobo| will start asking `B` to process it. As soon as the first
+row of `B`'s output is available, |bonobo| will start asking `C` to process it.
 
 While `B` and `C` are processing, `A` continues to generate data.
 
 This approach can be efficient, depending on your requirements, because you may rely on a lot of services that may be
 long to answer or unreliable, and you don't have to handle optimizations, parallelism or retry logic by yourself.
 
+.. note::
+
+    The default execution strategy uses threads, and makes it efficient to work on I/O bound tasks. It's in the plans
+    to have other execution strategies, based on subprocesses (for CPU-bound tasks) or `dask.distributed` (for big
+    data tasks that requires a cluster of computers to process in reasonable time).
+
+Graphs with divergence points (or forks)
+----------------------------------------
+
 .. graphviz::
 
     digraph {
@@ -55,6 +75,9 @@ In this case, any output row of `A`, will be **sent to both** `B` and `C` simult
 processing while `B` and `C` are working.
 
 
+Graph with convergence points (or merges)
+-----------------------------------------
+
 .. graphviz::
 
     digraph {
@@ -71,38 +94,23 @@ processing while `B` and `C` are working.
 Now, we feed `C` with both `A` and `B` output. It is not a "join", or "cartesian product". It is just two different
 pipes plugged to `C` input, and whichever yields data will see this data feeded to `C`, one row at a time.
 
+
 What is it not?
 :::::::::::::::
 
-**Bonobo** is not:
+|bonobo| is not:
 
 * A data science, or statistical analysis tool, which need to treat the dataset as a whole and not as a collection of
   independant rows. If this is your need, you probably want to look at `pandas <https://pandas.pydata.org/>`_.
 
 * A workflow or scheduling solution for independant data-engineering tasks. If you're looking to manage your sets of
   data processing tasks as a whole, you probably want to look at `airflow <https://airflow.incubator.apache.org/>`_.
-  Although there is no Bonobo extension yet that handles that, it does make sense to integrate Bonobo jobs in an airflow
-  (or other similar tool) workflow.
+  Although there is no |bonobo| extension yet that handles that, it does make sense to integrate |bonobo| jobs in an
+  airflow (or other similar tool) workflow.
 
 * A big data solution, `as defined by wikipedia <https://en.wikipedia.org/wiki/Big_data>`_. We're aiming at "small
   scale" data processing, which can be still quite huge for humans, but not for computers. If you don't know whether or
   not this is sufficient for your needs, it probably means you're not in the "big data" land.
 
 
-Where to jump next?
-:::::::::::::::::::
-
-If you did not run through it yet, we highly suggest that you go through the :doc:`tutorial </tutorial/index>` first.
-
-Then, you can jump to the following guides, in no particuliar order:
-
-.. toctree::
-    :maxdepth: 1
-
-    transformations
-    graphs
-    services
-    environment
-    purity
-
-
+.. include:: _next.rst