Update documentation for activating virtual environments in different shell (#11000)

## Add activation commands for fish shell and other alternative shells

While trying to use uv with fish shell, I encountered an issue as
`source .venv/bin/activate` didn't work. The documentation didn't
specify that fish shell requires using `source .venv/bin/activate.fish`
instead. I created issue #10986 to address this.

This PR improves the documentation by:
- Adding the correct activation command for fish shell: `source
.venv/bin/activate.fish`
- Adding the correct activation command for Nushell: `use
.venv\Scripts\activate.nu`
- Adding the correct activation command for Tcsh: `use
.venv/bin/activate.csh`

This will help users of alternative shells to properly activate their
virtual environments without encountering the same confusion I
experienced.

Fixes #10986

---------

Co-authored-by: Zanie Blue <[email protected]>

Author: CedricRaison

docs/pip/environments.md

@@ -57,6 +57,30 @@ The virtual environment can be "activated" to make its packages available:
     $ .venv\Scripts\activate
     ```
 
+!!! note
+
+    The default activation script on Unix is for POSIX compliant shells like `sh`, `bash`, or `zsh`.
+    There are additional activation scripts for common alternative shells.
+
+    === "fish"
+
+        ```console
+        $ source .venv/bin/activate.fish
+        ```
+
+    === "csh / tcsh"
+
+
+        ```console
+        $ source .venv/bin/activate.csh
+        ```
+
+    === "Nushell"
+
+        ```console
+        $ use .venv\Scripts\activate.nu
+        ```
+
 ## Using arbitrary Python environments
 
 Since uv has no dependency on Python, it can install into virtual environments other than its own.