docs: add cells_data documentation

Signed-off-by: Alessandro Comodi <acomodi@antmicro.com>

docs/dev_database/common/cell_data.rst

diff --git a/docs/dev_database/common/cell_data.rst b/docs/dev_database/common/cell_data.rst
new file mode 100644
index 0000000..07dbf8d
--- /dev/null
+++ b/docs/dev_database/common/cell_data.rst
@@ -0,0 +1,82 @@
+===============
+cell data files
+===============
+
+The *cell data* files are meant for specific primitives which have a common attribute format. The data contained in these files is generated/copied from the corresponding primitives' fuzzers.
+
+Naming convention
+-----------------
+
+The naming scheme for the cell data files is the following::
+
+    <primitive_name>_<data_file_type>.json
+
+Example files:
+
+    - ``gtpe2_common_attrs.json``
+    - ``gtpe2_channel_ports.json``
+
+File format
+-----------
+
+There are two main data file types:
+
+    - *Ports*
+    - *Attributes*
+
+Attributes files
+~~~~~~~~~~~~~~~~
+
+This is a JSON file containing a dictionary of parameters, each one with, at most, four attributes:
+
+    - Type: one of BIN, INT, STR, BOOL.
+    - Values: all possible values that this parameter can assume. In case of `BIN` types, the values list contains only the maximum value reachable.
+    - Digits: number of digits (or bits) required to enable a parameter.
+    - Encoding: This is present only for `INT` types of parameters. These reflect the actual encoding of the parameter value in the bit array.
+
+As an example of parameter please, refer to the following::
+
+    {
+        "PLL0_REFCLK_DIV": {
+            "type": "INT",
+            "values": [1, 2],
+            "encoding": [16, 0],
+            "digits": 5
+        },
+        "RXLPMRESET_TIME": {
+            "type": "BIN",
+            "values": [127],
+            "digits": 7
+        },
+        "RX_XCLK_SEL": {
+            "type": "STR",
+            "values": ["RXREC", "RXUSR"],
+            "digits": 1
+        },
+        "TX_LOOPBACK_DRIVE_HIZ": {
+            "type": "BOOL",
+            "values": ["FALSE", "TRUE"],
+            "digits": 1
+        },
+    }
+
+Ports files
+~~~~~~~~~~~
+
+This is a JSON file containing a dictionary of ports, each one with two attributes:
+
+    - Direction: Corresponds to the port directiona and can have the ``input`` or ``output`` values.
+    - Width: Indicates the width of the port bus.
+
+As an example of parameter please, refer to the following::
+
+    {
+        "CFGRESET": {
+            "direction": "input",
+            "width": 1
+        },
+        "CLKRSVD0": {
+            "direction": "input",
+            "width": 1
+        }
+    }

docs/dev_database/common/index.rst

diff --git a/docs/dev_database/common/index.rst b/docs/dev_database/common/index.rst
index 939999e..0817f9d 100644
--- a/docs/dev_database/common/index.rst
+++ b/docs/dev_database/common/index.rst
@@ -13,3 +13,4 @@
    tile_type
    ppips
    mask
+   cell_data