Provide the correct environment for doc build (#2643)

* Add riscv-isa-sim submodule

* Add cmake, bison, flex, pango, gdk-pixbuf-2.0, libgdk-pixbuf2.0-0, gdk-pixbuf-2.0, libgtk2.0-dev

* Update .readthedocs.yaml

.readthedocs.yaml

@@ -4,12 +4,23 @@
 
 version: 2
 
+submodules:
+  include:
+    - docs/riscv-isa/riscv-isa-manual
+
 build:
   os: "ubuntu-20.04"
   tools:
     python: "3.9"
     nodejs: "20"
     ruby: "3.3"
+  apt_packages:
+    - cmake
+    - bison
+    - flex
+    - libpango1.0-dev
+    - libgdk-pixbuf2.0-0
+    - libgtk2.0-dev
   jobs:
     post_install:
       - npm install docs/riscv-isa/riscv-isa-manual/dependencies