[Doc] Fix other links from gitlab to sphinx

ogstools/feflowlib/README.md

@@ -3,4 +3,4 @@
 See the documentation for more info:
 
 - [Documentation source](../../docs/user-guide/feflowlib.md)
-- [Documentation web site](https://ogstools.opengeosys.org/user-guide/feflowlib.html)
+- [Documentation web site](https://ogstools.opengeosys.org/stable/user-guide/feflowlib.html)

ogstools/meshlib/README.md

 See the documentation for more info:
 
 - [Documentation source](../../docs/user-guide/meshlib.md)
-- [Documentation web site](https://ogstools.opengeosys.org/user-guide/meshlib.html)
+- [Documentation web site](https://ogstools.opengeosys.org/stable/user-guide/meshlib.html)

ogstools/msh2vtu/README.md

@@ -3,4 +3,4 @@
 See the documentation for more info:
 
 - [Documentation source](../../docs/user-guide/msh2vtu.md)
-- [Documentation web site](https://ogstools.opengeosys.org/user-guide/msh2vtu.html)
+- [Documentation web site](https://ogstools.opengeosys.org/stable/user-guide/msh2vtu.html)

ogstools/plot/README.md

@@ -3,4 +3,4 @@
 See the documentation for more info:
 
 - [Documentation source](../../docs/user-guide/plot.md)
-- [Documentation web site](https://ogstools.opengeosys.org/user-guide/plot.html)
+- [Documentation web site](https://ogstools.opengeosys.org/stable/user-guide/plot.html)

ogstools/variables/README.md

@@ -3,4 +3,4 @@
 See the documentation for more info:
 
 - [Documentation source](../../docs/user-guide/variables.md)
-- [Documentation web site](https://ogstools.opengeosys.org/user-guide/variables.html)
+- [Documentation web site](https://ogstools.opengeosys.org/stable/user-guide/variables.html)