[External] [docs] Add fish version of the set-up command (#39266)

[External] [docs] Add `fish` version of the set-up command

Co-authored-by: Vasilis Themelis <vdthemelis@gmail.com>
Closes modularml#2423
MODULAR_ORIG_COMMIT_REV_ID: 376a924f470e84b9f1775382ee452386fbe3e281

Signed-off-by: Manuel Saelices <msaelices@gmail.com>

docs/manual/get-started/index.md

@@ -129,6 +129,17 @@ If you already have the `modular` tool,
         && source ~/.zshrc
       ```
 
+      </TabItem>
+      <TabItem value="fish" label="fish">
+
+      If you're using fish, run this command:
+
+      ```sh
+      set MOJO_PATH (modular config mojo.path) \
+        && set -Ux MODULAR_HOME $HOME/.modular \
+        && fish_add_path $MOJO_PATH/bin
+      ```
+
       </TabItem>
     </Tabs>