doc: add URLs to CDF-1,2,5 format specifications

Author: wkliao

docs/nc4_vs_pnetcdf.md

@@ -18,13 +18,17 @@ operational modes.
   + Classic file format (CDF-1) -- The ESDS Community Standard defined the file format
     to be used in the NetCDF user community in 1989. The file header bears a
     signature of character string 'CDF-1' and now is commonly referred to as
-    'CDF-1' file format.
+    [CDF-1](https://parallel-netcdf.github.io/doc/c-reference/pnetcdf-c/CDF_002d1-file-format-specification.html)
+    file format.
     * 'CDF-2' format -- The CDF-1 format was later extended to support large
       file size (i.e.  larger than 2GB) in 2004. See its specification in
       [ESDS-RFC-011v2.0](https://cdn.earthdata.nasa.gov/conduit/upload/496/ESDS-RFC-011v2.00.pdf).
       Because its file header bears a signature of 'CDF-2' and the format is
-      also commonly referred to as 'CDF-2' format.
-    * 'CDF-5' format -- The CDF-2 format was extended by PnetCDF developer team
+      also commonly referred to as
+      [CDF-2](https://parallel-netcdf.github.io/doc/c-reference/pnetcdf-c/CDF_002d2-file-format-specification.html)
+      format.
+    * [CDF-5](https://parallel-netcdf.github.io/doc/c-reference/pnetcdf-c/CDF_002d5-file-format-specification.html)
+      format -- The CDF-2 format was extended by PnetCDF developer team
       in 2009 to support large variables and additional large data types, such
       as 64-bit integer.
   + HDF5-based file format -- Starting from its version 4.0.0, NetCDF includes

docs/source/tutorial/basic.rst

@@ -20,8 +20,12 @@ Creating/Opening/Closing a netCDF file
  This is also the method used to open an existing netCDF file. If the file is
  open for write access (mode='w', 'r+' or 'a'), you may write any type of data
  including new dimensions, variables and attributes. Currently, netCDF files
- can be created in classic formats, specifically the formats of CDF-1, 2, and
- 5. When creating a new file, the format may be specified using the format
+ can be created in classic formats, specifically the formats of
+ `CDF-1 <https://parallel-netcdf.github.io/doc/c-reference/pnetcdf-c/CDF_002d1-file-format-specification.html>`_,
+ `CDF-2 <https://parallel-netcdf.github.io/doc/c-reference/pnetcdf-c/CDF_002d2-file-format-specification.html>`_,
+ and
+ `CDF-5 <https://parallel-netcdf.github.io/doc/c-reference/pnetcdf-c/CDF_002d5-file-format-specification.html>`_.
+ When creating a new file, the format may be specified using the format
  keyword in the ``File`` constructor. The default format is CDF-1. To see how a
  given file is formatted, you can examine the ``file_format`` attribute.
  Closing the netCDF file is accomplished via the :meth:`File.close` method of

docs/source/tutorial/datatypes.rst

@@ -4,7 +4,10 @@ Data type
 
 NetCDF Variable Data Types
  The following table gives the netCDF external data types defined in CDF-1 and
- CDF-2 and the corresponding type constants for defining variables in the
+ `CDF-1 <https://parallel-netcdf.github.io/doc/c-reference/pnetcdf-c/CDF_002d1-file-format-specification.html>`_
+ and
+ `CDF-2 <https://parallel-netcdf.github.io/doc/c-reference/pnetcdf-c/CDF_002d2-file-format-specification.html>`_
+ and the corresponding type constants for defining variables in the
  python interface. All these data types have direct numpy quivalent.
 
 
@@ -26,7 +29,10 @@ NetCDF Variable Data Types
       +-------+----------------+-------+----------------------------------------+---------------------+
 
 
- Additional data types supported in CDF-5 format:
+ Additional data types supported in
+ `CDF-5 <https://parallel-netcdf.github.io/doc/c-reference/pnetcdf-c/CDF_002d5-file-format-specification.html>`_.
+ format:
+
 
       +---------------------+----------------+-------+----------------------------------------+---------------------+
       | Type                | C #define      | Bits  | Intent of use                          | Numpy Equivalent    |
@@ -41,3 +47,4 @@ NetCDF Variable Data Types
       +---------------------+----------------+-------+----------------------------------------+---------------------+
       | unsigned long long  | NC_UINT64      | 64    | unsigned 8-byte integer                | np.uint64 or 'u8'   |
       +---------------------+----------------+-------+----------------------------------------+---------------------+
+

src/pnetcdf/_File.pyx

@@ -52,10 +52,15 @@ cdef class File:
         :param str format: [Optional] underlying file format. Only relevant
             when creating a new file.
 
-            - ``NETCDF3_64BIT_OFFSET`` or ``NC_64BIT_OFFSET``: CDF-2 format
-            - ``NETCDF3_64BIT_DATA`` or ``NC_64BIT_DATA``: CDF-5 format
+            - ``NETCDF3_64BIT_OFFSET`` or ``NC_64BIT_OFFSET``:
+              `CDF-2 <https://parallel-netcdf.github.io/doc/c-reference/pnetcdf-c/CDF_002d2-file-format-specification.html>`_
+              format
+            - ``NETCDF3_64BIT_DATA`` or ``NC_64BIT_DATA``:
+              `CDF-5 <https://parallel-netcdf.github.io/doc/c-reference/pnetcdf-c/CDF_002d5-file-format-specification.html>`_
+              format
             - ``NETCDF3_CLASSIC`` or `None` defaults to default file format
-              (CDF-1 format)
+              (`CDF-1 <https://parallel-netcdf.github.io/doc/c-reference/pnetcdf-c/CDF_002d1-file-format-specification.html>`_
+              format)
 
         :param comm: [Optional]
             MPI communicator to use for file access. `None` defaults to