doc: update rpc docs before pushing to readme

Now that our rpc documenation is generated from schema, we need to build
the docs before trying to update the rdme contents.  The goal is to fix
the following issue with the rdme-rpc-sync workflow:

Run python .github/scripts/sync-rpc-cmds.py
lightning-addgossip		lightning-addgossip.7.md
Traceback (most recent call last):
  File .github/scripts/sync-rpc-cmds.py, line 92, in <module>
    main()
  File .github/scripts/sync-rpc-cmds.py, line 82, in main
    with open(doc/ + file) as f:
FileNotFoundError: [Errno 2] No such file or directory: 'doc/lightning-addgossip.7.md'

.github/workflows/readme-rpc-sync.yml

@@ -22,6 +22,20 @@ jobs:
     - name: Install requests module
       run: python -m pip install requests
 
+    - name: Install dependencies
+      run: bash -x .github/scripts/setup.sh
+
+    - name: Build
+      run: |
+        ./configure --enable-debugbuild
+        make -j $(nproc)
+        make -j $(nproc) check-gen-updated
+
+    - name: Make docs
+      run: |
+        make doc-all
+        make check-doc
+
     - name: Set environment variable and run
       env:
         README_API_KEY: ${{ secrets.README_API_KEY }}