outdated building instructions, update documentation.md

The current instructions for building the Nix manual include a command that doesn't work as described. Specifically:

```
nix build .#nix^doc
```

Running this command results in the error:

```
error: derivation '/nix/store/hddqxzfqgx2fhj8q66ss3idym7pk7aj1-nix-2.26.0pre20250107_383ab87.drv' does not have wanted outputs 'doc'
```

However, this command works if you specify the Nix version explicitly, such as:

```
nix build nix/2.24.11#nix^doc
```

Additionally, these commands are run within the Nix root directory. 

However, the nix build .#nix^doc command does work when run from the nixpkgs directory and generates the NixOS manual.

I'm not sure if I'm missing something. Is the `nix^doc` supposed to be added somehow to flake outputs?

The incremental build section does not work since as make has been decommissioned in favor of Meson. Should this be simply deleted?

Author: allrealmsoflife

doc/manual/source/development/documentation.md

@@ -19,10 +19,11 @@ nix-build -E '(import ./.).packages.${builtins.currentSystem}.nix.doc'
 or
 
 ```console
-nix build .#nix^doc
+nix build .#nix-manual
 ```
 
-and open `./result-doc/share/doc/nix/manual/index.html`.
+and open `./result/share/doc/nix/manual/index.html`.
+
 
 To build the manual incrementally, [enter the development shell](./building.md) and run: