fix(ci): install fish from the official 4.x PPA

Ubuntu 24.04's default repo only carries fish 3.7, but
docs/manual/06-dependency-catalog.md states fish >= 4.0 as a
project dependency. ppa:fish-shell/release-4 is the fish-shell
maintainers' own PPA and explicitly supports 24.04.

.github/workflows/build-docs.yml

@@ -26,6 +26,9 @@ jobs:
 
       - name: Install dependencies
         run: |
+          sudo apt-get update -qq
+          sudo apt-get install -y software-properties-common
+          sudo add-apt-repository -y ppa:fish-shell/release-4
           sudo apt-get update -qq
           sudo apt-get install -y pandoc python3-yaml fish