Fix PowerShell code blocks (#13511)

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## Summary

The PowerShell prompt is not `$`, so it is not detected as a
`Generic.Prompt` token by Pygments lexers. Therefore, the JavaScript
code does not strip the prompt when copying from PowerShell code blocks,
such as
[here](https://docs.astral.sh/uv/getting-started/installation/#__tabbed_5_2).

Other places in the docs have removed the prompt completely to address
this issue:
* https://docs.astral.sh/uv/guides/projects/#__tabbed_1_2
* https://docs.astral.sh/uv/guides/integration/jupyter/#__tabbed_1_2

This PR updates the PowerShell prompt to `PS>` and changes the code
fence language to `pwsh-session` to match the lexer name from
[Pygments](https://pygments.org/docs/lexers/#pygments.lexers.shell.PowerShellSessionLexer).
This allows the prompt to be correctly detected as a `Generic.Prompt`
token and is stripped during copy.

Related: https://github.com/astral-sh/uv/pull/12520

docs/pip/environments.md

@@ -53,8 +53,8 @@ The virtual environment can be "activated" to make its packages available:
 
 === "Windows"
 
-    ```console
-    $ .venv\Scripts\activate
+    ```pwsh-session
+    PS> .venv\Scripts\activate
     ```
 
 !!! note