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

[David Spickett 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
    environment:
 
    ```bash
    % python3 -m venv .venv
    % . .venv/bin/activate
    % pip install git+https://github.com/llvm/llvm-project.git#subdirectory=llvm/utils/lit
    % lit --version
-   lit 0.8.0dev
+   lit 20.0.0dev
+   ```
+
+   Installing the official Python release of lit in a Python virtual
+   environment would also work. This will install the most recent 
+   release of lit:
----------------
DavidSpickett wrote:

I would say `could also work`. And add:
```
Note that recent tests may rely on features not in the latest released lit. If in doubt, try one of the previous methods.
```

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