[llvm] [llvm][Docs] Update guide to include ``pip install lit`` (PR #106526)

[Anatoly Trosinenko via llvm-commits]

================
@@ -9,18 +9,29 @@ Quickstart
 
    ```bash
    % <path to llvm build>/bin/llvm-lit --version
-   lit 0.8.0dev
+   lit 20.0.0dev
    ```
 
-   An alternative is installing it as a python package in a python virtual
+   An alternative is installing it as a Python package in a python virtual
----------------
atrosinenko wrote:

Probably missed lower-case "python" (not being a command name).

```suggestion
   An alternative is installing it as a Python package in a Python virtual
```

https://github.com/llvm/llvm-project/pull/106526