test chatgtp proposed link pattern

For links between sections of different MyST Markdown files,
chatgtp proposes a pattern like

```chapter02.md

some text

```

as an anchor and

```chapter03.md

As shown [earlier]{chapter02.md#special} ...
```

as use of this anchor.  As a test, this pattern is used between
file `expressions_and_assignments.md` which to link to file
`program_units_and_procedures.md`, section modules.  The local
build only links to the top of the corresponding index, maybe
a build which uses the setup of the fortran-lang.org website
however retains the wanted link to the very spot.

Signed-off-by: Norwid Behrnd <[email protected]>

Author: nbehrnd

source/learn/f95_features/expressions_and_assignments.md

@@ -113,7 +113,7 @@ vector3 =(matrix .times. vector1) + vector2
 
 the two expressions are equivalent only if appropriate parentheses are
 added as shown. In each case there must be defined, in a
-[module](modules),
+[module](program_units_and_procedures.md#modules),
 procedures defining the operator and assignment, and corresponding
 operator-procedure association, as follows:
 

source/learn/f95_features/program_units_and_procedures.md

@@ -76,8 +76,8 @@ The names of program units and external procedures are *global*, and the
 names of implied-DO variables have a scope of the statement that
 contains them.
 
-(modules)=
-## Modules
+
+## Modules {#modules}
 
 Modules are used to package