Documentation: Improve nix related commands

This patch fixes `nix develop` commands not using proper paths, uses
the serenity root directory as an assumption for all commands and
adds variants of nix commands to use a custom entry point, for example
a shell.

Author: networkException

Documentation/BuildInstructionsLadybird.md

@@ -34,13 +34,22 @@ On openSUSE:
 sudo zypper install cmake libglvnd-devel ninja qt6-base-devel qt6-multimedia-devel qt6-tools-devel qt6-wayland-devel ccache
 ```
 
-On Nix/NixOS:
-```
-nix-shell ladybird.nix
+On Nix/NixOS (using your host nixpkgs):
+```bash
+# From /path/to/serenity
+nix-shell Ladybird/ladybird.nix
+
+# With a custom entrypoint, for example your favorite shell
+nix-shell --command bash Ladybird/ladybird.nix
 ```
+
 You can also use the nix flake in `Toolchain`:
-```
-nix develop Toolchain/#ladybird
+```bash
+# From /path/to/serenity
+nix develop ./Toolchain#ladybird
+
+# With a custom entrypoint, for example your favorite shell
+nix develop ./Toolchain#ladybird --command bash
 ```
 
 On macOS:

Documentation/BuildInstructionsOther.md

@@ -27,16 +27,22 @@ apt-get install curl cmake libmpc-devel gmp-devel e2fsprogs libmpfr-devel ninja-
 
 ## NixOS
 
-You can use the `nix-shell` script [`Toolchain/serenity.nix`](../Toolchain/serenity.nix) to set up the environment:
+You can use the `nix-shell` script [`Toolchain/serenity.nix`](../Toolchain/serenity.nix) to set up the environment (using your host nixpkgs):
 
 ```console
 nix-shell Toolchain/serenity.nix
+
+# With a custom entrypoint, for example your favorite shell
+nix-shell --command bash Toolchain/serenity.nix
 ```
 
 or you can use the nix flake [`Toolchain/flake.nix`](../Toolchain/flake.nix) instead:
 
 ```console
-nix develop Toolchain
+nix develop ./Toolchain#
+
+# With a custom entrypoint, for example your favorite shell
+nix develop ./Toolchain# --command bash
 ```
 
 You can also save this environment to a profile: