Nix: update and add documentation for nix subcommands (#9959)

* Nix: update and add documentation for nix subcommands

Updates the documentation on the new unstable Nix command & subcommands,
and adds a separate page for the Classic Nix commands.

---------

Co-authored-by: K.B.Dharun Krishna <kbdharunkrishna@gmail.com>
Co-authored-by: axtloss <axtlos@getcryst.al>
Co-authored-by: CleanMachine1 <78213164+CleanMachine1@users.noreply.github.com>

pages/common/nix3-build.md

@@ -0,0 +1,21 @@
+# nix build
+
+> Build a Nix expression (downloading from the cache when possible).
+> See also: `tldr nix-build`. See `tldr nix3 flake` for information about flakes.
+> More information: <https://nixos.org/manual/nix/stable/command-ref/new-cli/nix3-build.html>.
+
+- Build a package from nixpkgs, symlinking the result to `./result`:
+
+`nix build {{nixpkgs#pkg}}`
+
+- Build a package from a flake in the current directory, showing the build logs in the process:
+
+`nix build -L {{.#pkg}}`
+
+- Build the default package from a flake in some directory:
+
+`nix build {{./path/to/directory}}`
+
+- Build a package without making the `result` symlink, instead printing the store path to the stdout:
+
+`nix build --no-link --print-out-paths`