build: configure dependabot uv updates for dev lockfiles (#4324)

Dev and documentation dependencies moved from `/docs` to `uv.lock`
files in `/dev` and `/sphinxdocs/dev`. Without `uv` ecosystem updates
configured, Dependabot only opens `pip` updates for `requirements.txt`,
leaving `uv.lock` out of sync.

Configure weekly `uv` ecosystem updates for `/dev` and
`/sphinxdocs/dev`, and remove the obsolete `/docs` directory from `pip`
updates.

.github/dependabot.yml

diff --git a/.github/dependabot.yml b/.github/dependabot.yml
index 8aa2155..19877db 100644
--- a/.github/dependabot.yml
+++ b/.github/dependabot.yml
@@ -12,8 +12,15 @@
   - package-ecosystem: "pip"
     directories:
       # Maintain dependencies for our tools
-      - "/docs"
       - "/tools/publish"
     schedule:
       interval: "weekly"
     open-pull-requests-limit: 3
+
+  - package-ecosystem: "uv"
+    directories:
+      - "/dev"
+      - "/sphinxdocs/dev"
+    schedule:
+      interval: "weekly"
+    open-pull-requests-limit: 3