Version Switcher Part 3 (#47019)

## Why are these changes needed?

A few small follow ups for the version switcher:
- move constants to inside the versions JSON generation function
- updated height of version switcher to be relative so it scales up with
zoom level
- changed `overflow` to `overflow-y` to stop the extra horizontal scroll
from showing up
Before:
![Screenshot 2024-08-08 at 9 56
01 AM](https://github.com/user-attachments/assets/ac94b727-ca20-4c3b-8488-347549d19eca)
After:
![Screenshot 2024-08-08 at 10 02
21 AM](https://github.com/user-attachments/assets/05813997-bee1-480a-bbcb-e4f02ac88a99)

Signed-off-by: cristianjd <cristian.j.derr@gmail.com>

doc/source/_static/css/custom.css

@@ -393,8 +393,8 @@ table.autosummary tr > td:first-child > p > a > code > span {
 
 /* Prevent the the PyData theme Version Switcher from getting too large */
 .version-switcher__menu {
-  max-height: 496px;
-  overflow: scroll;
+  max-height: 40rem;
+  overflow-y: scroll;
 }
 
 /* Hide the RTD version switcher since we are using PyData theme one */

doc/source/custom_directives.py

@@ -1256,14 +1256,6 @@ def pregenerate_example_rsts(
             )
 
 
-ray_prefix = "ray-"
-min_version = "1.11.0"
-repo_url = "https://github.com/ray-project/ray.git"
-static_dir_name = "_static"
-version_json_filename = "versions.json"
-dereference_suffix = "^{}"
-
-
 def generate_version_url(version):
     return f"https://docs.ray.io/en/{version}/"
 
@@ -1273,6 +1265,13 @@ def generate_versions_json():
     and generates the JSON needed for the version switcher
     """
 
+    ray_prefix = "ray-"
+    min_version = "1.11.0"
+    repo_url = "https://github.com/ray-project/ray.git"
+    static_dir_name = "_static"
+    version_json_filename = "versions.json"
+    dereference_suffix = "^{}"
+
     version_json_data = []
 
     # Versions that should always appear at the top