In a few places, replace “Octave” by “GNU Octave”

This is a revert 683011b4.

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.

_includes/download-snapshot.html

@@ -15,7 +15,7 @@
   <section class="dynare_section" id="snapshot_content1">
     <p>
       Compatible with Windows (7, 8.1, 10), MATLAB (R2009b and later) and
-      Octave (5.2.0 only,
+      GNU Octave (5.2.0 only,
       <a href="https://ftp.gnu.org/gnu/octave/windows/octave-5.2.0_1-w64-installer.exe">64-bit</a>
       or <a href="https://ftp.gnu.org/gnu/octave/windows/octave-5.2.0_1-w32-installer.exe">32-bit</a>)
     </p>
@@ -39,7 +39,7 @@
   <section class="dynare_section" id="snapshot_content2">
     <p>
       Compatible with macOS (10.11 and later, earlier versions may work but
-      have not been tested), any 64-bit MATLAB, and Octave (4.4.1 installed
+      have not been tested), any 64-bit MATLAB, and GNU Octave (4.4.1 installed
       via <a href="https://octave-app.org">https://octave-app.org</a>). <b>NB</b>:
       This package is unsigned; to run it, you need to right click on it and
       select "Open" from the menu that pops up.

_includes/download-stable.html

@@ -15,7 +15,7 @@
   <section class="dynare_section" id="stable_content1">
     <p>
       Compatible with Windows (7, 8.1, 10), MATLAB (R2007b and later) and
-      Octave (4.4.1 only,
+      GNU Octave (4.4.1 only,
       <a href="https://ftp.gnu.org/gnu/octave/windows/octave-4.4.1-w64-installer.exe">64-bit</a>
       or <a href="https://ftp.gnu.org/gnu/octave/windows/octave-4.4.1-w32-installer.exe">32-bit</a>)
     </p>

index.md

@@ -47,7 +47,7 @@ We organize a Summer School and a Conference every year (applications every Spri
 </div>
 <div class="dynare_home_page_about_column" markdown="1">
 <h3>Platforms</h3>
-Dynare runs on top of MATLAB, [Octave](https://www.gnu.org/software/octave/), and, in the not too distant future, [Julia](https://julialang.org/)!
+Dynare runs on top of MATLAB, [GNU Octave](https://www.octave.org/), and, in the not too distant future, [Julia](https://julialang.org/)!
 </div>
 </div>