[skip-ci] Add doxygen doc for the tree tutorials

tutorials/tree/index.md

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+\addtogroup tutorial_tree
+
+@{
+[TTree](classTTree.html) represents a columnar dataset. Used for example by all LHC (Large Hadron Collider) experiments. Trees are optimized for reduced disk space and selecting, high-throughput columnar access with reduced memory usage.
+
+...
+
+In a nutshell:
+~~~{.cpp}
+auto file = TFile::Open("myfile.root", "recreate");
+auto tree = new TTree("mytree", "A tree with my data");
+...
+~~~
+
+Explore the examples below or go to [TTree class reference](classTTree.html).
+
+## Tutorials sorted after groups
+- [Basic Usage](\ref basic)
+- [Copying Trees](\ref copy)
+- [Collection Classes](\ref collections)
+- [Advanced examples](\ref advanced)
+- [Graphics](\ref graphics)
+- [More tutorials](\ref other)
+
+
+[List of all tutorials](\ref alltutorials)
+\anchor basic
+## Basic usage
+
+These examples show how data can be stored to and read back from TTree.
+
+| **Tutorial** | **Description** |
+|------|-----------------|
+| tree101_basic.C | Basic TTree usage. |
+| tree102_basic.C | A variant of tree101_basic.C |
+| tree103_tree.C | Simple Event class example |
+| tree104_tree.C | A variant of hsimple.C but using a [TTree](classTTree.html) instead of a [TNtuple](classTNtuple.html) |
+| tree105_tree.C | Illustrates how to make a tree from variables or arrays in a C struct - without a dictionary, by creating the branches for builtin types (int, float, double) and arrays explicitly |
+| tree106_tree.C | The same as `tree105_tree.C`, but uses a class instead of a C-struct |
+| tree107_tree.C | Example of a tree where branches are variable length arrays |
+| tree108_tree.C | This example writes a tree with objects of the class `Event` |
+| tree109_friend.C | Illustrates how to use tree friends |
+| tree113_getval.C | Illustrates how to retrieve tree variables in arrays |
+| tree114_circular.C | Example of a circular Tree. Circular Trees are interesting in online real time environments to store the results of the last maxEntries events |
+
+\anchor copy
+## Copying Trees
+
+These examples shows how to copy the content of trees.
+
+| **Tutorial** | **Description** |
+|------|-----------------|
+| tree110_copy.C | Copy a subset of a tree to a new tree |
+| tree111_copy.C | Copy a subset of a tree to a new tree, one branch in a separate file |
+| tree112_copy.C | Copy a subset of a tree to a new tree, selecting entries |
+
+
+\anchor collections
+## Collection Classes
+
+These examples show how to write and read several collection classes (std::vector, [TClonesArray](classTClonesArray.html),...) to trees.
+
+| **Tutorial** || **Description** |
+|------|-----------------|
+| tree120_ntuple.C | Read a [TNtuple](classTNtuple.html) from a file and performs a simple analysis |
+| tree121_hvector.C | Write and read STL vectors in a tree |
+| tree122_vector3.C | Write and read a `Vector3` class in a tree|
+| tree123_clonesarray.C | Write and read a [TClonesArray](classTClonesArray.html) to a tree |
+
+
+\anchor advanced
+## More advanced examples
+
+These examples shows a couple of more advanced examples using complex data.
+
+| **Tutorial** | **Description** |
+|------|-----------------|
+| tree130_jets.C | Usage of a tree using the `JetEvent` class. The `JetEvent` class has several collections ([TClonesArray](classTClonesArray.html)) and other collections ([TRefArray](classTRefArray.html)) referencing objects in the [TClonesArrays](classTClonesArray.html) |
+| tree131_clones_event.C | Example to write & read a Tree built with a complex class inheritance tree. It demonstrates usage of inheritance and TClonesArrays |
+
+
+\anchor graphics
+## Graphics
+
+These tutorials show how to generate complex graphics from [TNtuple](classTNtuple.html) and/or [TTree](classTTree.html).
+
+| **Tutorial** | **Description** |
+|------|-----------------|
+| tree140_spider.C | Example of a [TSpider](classTSpider.html) plot generated from a [TNtuple](classTNtuple.html) (from hsimple.root) |
+| tree141_parallelcoord.C | Illustrate the use of the [TParallelCoord](classTParallelCoord.html) class (from a [TNtuple](classTNtuple.html)) |
+| tree142_parallelcoordtrans.C | It displays [TParallelCoord](classTParallelCoord.html) from the same dataset twice. The first time without transparency and the second time with transparency |
+| tree143_drawsparse.C | Convert a [THnSparse](classTHnSparse.html) to a [TTree](classTTree.html) using efficient iteration through the [THnSparse](classTHnSparse.html) and draw it using [TParallelCoord](classTParallelCoord.html) |
+
+
+\anchor other
+## More tutorials
+
+A collection of other examples which showcases other usecases 
+| **Tutorial** | **Description** |
+|--------------|-----------------|
+| tree200_temperature.C | Illustrates how to use the highlight mode with trees, using a temperature dataset from Prague between 1775 and 2004 |
+| tree201_histograms.C | Save histograms in tree branches |
+| tree202_benchmarks.C | Benchmark comparing row-wise and column-wise storage performance |
+| tree500_cernbuild.C | Read data (CERN staff) from an ascii file and create a root file with a tree |
+| tree501_cernstaff.C | Playing with a tree containing variables of type character |
+| tree502_staff.C | Create a plot of the data in `cernstaff.root` |
+
+
+\anchor alltutorials
+@}