Nix docs: remove with lib; from example code

Following [Best Practices](https://nix.dev/guides/best-practices#with-scopes),
`with` is a problematic language construction and should be avoided.

Usually it is employed like a "factorization": `[ X.A X.B X.C X.D ]` is written
`with X; [ A B C D ]`.

However, as shown in the link above, the syntatical rules of `with` are not so
intuitive, and this "distributive rule" is very selective, in the sense that
`with X; [ A B C D ]` is not equivalent to `[ X.A X.B X.C X.D ]`.

However, this factorization is still useful to "squeeze" some code, especially
in lists like `meta.maintainers`.

On the other hand, it becomes less justifiable in bigger scopes. This is
especially true in cases like `with lib;` in the top of expression and in sets
like `meta = with lib; { . . . }`.

That being said, this patch removes most of example code in the current
documentation.

The exceptions are, for now
- doc/functions/generators.section.md
- doc/languages-frameworks/coq.section.md

because, well, they are way more complicated, and I couldn't parse them
mentally - yet another reason why `with` should be avoided!

doc/languages-frameworks/python.section.md

@@ -130,12 +130,12 @@ buildPythonPackage rec {
     hypothesis
   ];
 
-  meta = with lib; {
+  meta = {
     changelog = "https://github.com/pytest-dev/pytest/releases/tag/${version}";
     description = "Framework for writing tests";
     homepage = "https://github.com/pytest-dev/pytest";
-    license = licenses.mit;
-    maintainers = with maintainers; [ domenkozar lovek323 madjar lsix ];
+    license = lib.licenses.mit;
+    maintainers = with lib.maintainers; [ domenkozar lovek323 madjar lsix ];
   };
 }
 ```
@@ -312,7 +312,7 @@ python3Packages.buildPythonApplication rec {
     python-daemon
   ];
 
-  meta = with lib; {
+  meta = {
     # ...
   };
 }
@@ -899,12 +899,12 @@ buildPythonPackage rec {
     "toolz.dicttoolz"
   ];
 
-  meta = with lib; {
+  meta = {
     changelog = "https://github.com/pytoolz/toolz/releases/tag/${version}";
     homepage = "https://github.com/pytoolz/toolz";
     description = "List processing tools and functional utilities";
-    license = licenses.bsd3;
-    maintainers = with maintainers; [ fridh ];
+    license = lib.licenses.bsd3;
+    maintainers = with lib.maintainers; [ fridh ];
   };
 }
 ```
@@ -1034,12 +1034,12 @@ buildPythonPackage rec {
     pytest
   ];
 
-  meta = with lib; {
+  meta = {
     changelog = "https://github.com/blaze/datashape/releases/tag/${version}";
     homepage = "https://github.com/ContinuumIO/datashape";
     description = "A data description language";
-    license = licenses.bsd2;
-    maintainers = with maintainers; [ fridh ];
+    license = lib.licenses.bsd2;
+    maintainers = with lib.maintainers; [ fridh ];
   };
 }
 ```
@@ -1084,12 +1084,12 @@ buildPythonPackage rec {
     libxslt
   ];
 
-  meta = with lib; {
+  meta = {
     changelog = "https://github.com/lxml/lxml/releases/tag/lxml-${version}";
     description = "Pythonic binding for the libxml2 and libxslt libraries";
     homepage = "https://lxml.de";
-    license = licenses.bsd3;
-    maintainers = with maintainers; [ sjourdois ];
+    license = lib.licenses.bsd3;
+    maintainers = with lib.maintainers; [ sjourdois ];
   };
 }
 ```
@@ -1155,12 +1155,12 @@ buildPythonPackage rec {
   # Tests cannot import pyfftw. pyfftw works fine though.
   doCheck = false;
 
-  meta = with lib; {
+  meta = {
     changelog = "https://github.com/pyFFTW/pyFFTW/releases/tag/v${version}";
     description = "A pythonic wrapper around FFTW, the FFT library, presenting a unified interface for all the supported transforms";
     homepage = "http://hgomersall.github.com/pyFFTW";
-    license = with licenses; [ bsd2 bsd3 ];
-    maintainers = with maintainers; [ fridh ];
+    license = with lib.licenses; [ bsd2 bsd3 ];
+    maintainers = with lib.maintainers; [ fridh ];
   };
 }
 ```
@@ -1530,12 +1530,12 @@ buildPythonPackage rec {
     wheel
   ];
 
-  meta = with lib; {
+  meta = {
     changelog = "https://github.com/pytoolz/toolz/releases/tag/${version}";
     homepage = "https://github.com/pytoolz/toolz/";
     description = "List processing tools and functional utilities";
-    license = licenses.bsd3;
-    maintainers = with maintainers; [ fridh ];
+    license = lib.licenses.bsd3;
+    maintainers = with lib.maintainers; [ fridh ];
   };
 }
 ```