Manual: in a few places, replace “Octave” by “GNU Octave”

This is a partial revert of 8fa4c483.

Actually, the official name is “GNU Octave”, and it is easier to find it under
that name in search engines. So use the full name at a few prominent places,
and use the shorter “Octave” everywhere else.

doc/manual/source/installation-and-configuration.rst

@@ -15,7 +15,7 @@ are necessary in that case.
 In order to run Dynare, you need one of the following:
 
 * MATLAB version 7.9 (R2009b) or above;
-* Octave version 4.2.1 or above, with the statistics package from
+* GNU Octave version 4.2.1 or above, with the statistics package from
   `Octave-Forge`_. Note however that the Dynare installers for Windows and
   macOS require a more specific version of Octave, as indicated on the download
   page.

doc/manual/source/introduction.rst

@@ -55,7 +55,7 @@ manual. Part of Dynare is programmed in C++, while the rest is written
 using the `MATLAB`_ programming language. The latter implies that
 commercially-available MATLAB software is required in order to run
 Dynare. However, as an alternative to MATLAB, Dynare is also able to
-run on top of `Octave`_ (basically a free clone of MATLAB): this
+run on top of `GNU Octave`_ (basically a free clone of MATLAB): this
 possibility is particularly interesting for students or institutions
 who cannot afford, or do not want to pay for, MATLAB and are willing
 to bear the concomitant performance loss.
@@ -122,7 +122,7 @@ https://www.dynare.org.
 
 
 .. _MATLAB: https://www.mathworks.com/products/matlab/
-.. _Octave: https://www.octave.org/
+.. _GNU Octave: https://www.octave.org/
 .. _CEPREMAP: https://www.cepremap.fr/
 .. _web forum: https://forum.dynare.org/
 .. _official Dynare website: https://www.dynare.org/