docs: Move to standard folder naming

I also like docs/ more than doc/, but it's the standard in Qibo, and this is valuable

doc/source/getting-started/index.rst

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+Getting started
+===============
+
+In this section we present the basic aspects of the Qibotn design and provide installation instructions.
+Please visit the following sections to understand how ``qibotn`` works.
+
+.. toctree::
+    :maxdepth: 1
+
+    installation
+    quickstart

doc/source/getting-started/installation.rst

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+Installation instructions
+=========================
+
+QiboTN can be installed directly from the source repository on Github:
+
+.. code-block::
+
+    git clone https://github.com/qiboteam/qibotn.git
+    cd qibotn
+    poetry install

doc/source/getting-started/quickstart.rst

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+Quick start
+===========
+
+Setting the backend
+"""""""""""""""""""
+QiboTN supports two backends cutensornet (using CuQuantum library) and Quimbbackend (using Quimb library) for tensor network based simulations. The backend can be set using the following command line.
+
+For CuQuantum library,
+
+.. testcode::
+   qibo.set_backend(backend="qibotn", platform="cutensornet", runcard=computation_settings)
+..
+
+and for Quimb library
+
+.. testcode::
+   qibo.set_backend(
+       backend="qibotn", platform="QuimbBackend", runcard=computation_settings
+   )
+..
+
+Setting the runcard
+"""""""""""""""""""
+Basic structure of runcard is
+
+.. testcode::
+   computation_settings = {
+       "MPI_enabled": False,
+       "MPS_enabled": False,
+       "NCCL_enabled": False,
+       "expectation_enabled": {
+           "pauli_string_pattern": "IXZ",
+       },
+   }
+..
+
+Basic example
+"""""""""""""
+
+.. testcode::
+   # Construct the circuit
+   c = Circuit(2)
+   # Add some gates
+   c.add(gates.H(0))
+   c.add(gates.H(1))
+
+   # Execute the circuit and obtain the final state
+   result = c()
+
+   print(result.state())
+
+..