Add reference to gromax manual for xtc

doc/source/formats/reference/xtc.rst

@@ -7,8 +7,9 @@ XTC (GROMACS compressed trajectory file)
 
 .. include:: classes/XTC.txt
 
-The GROMACS XTC trajectory compresses data with reduced precision (3 decimal places by default). MDAnalysis can only read coordinates from these files. See :ref:`TRR-format` for uncompressed files that provide velocity and force information.
+The `GROMACS XTC`_ trajectory compresses data with reduced precision (3 decimal places by default). MDAnalysis can only read coordinates from these files. See :ref:`TRR-format` for uncompressed files that provide velocity and force information.
 
+.. _`GROMACS XTC`: https://manual.gromacs.org/current/reference-manual/file-formats.html#xtc
 
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