CI: exclude doc/{design-thoughts,internals} from spell check

as those directories do contain many documents written in French,
codespell is catching a lot of false positives scanning them.

.github/workflows/codespell.yml

@@ -15,5 +15,5 @@ jobs:
     - uses: codespell-project/codespell-problem-matcher@v1
     - uses: codespell-project/actions-codespell@master
       with:
-        skip: CHANGELOG,Makefile,*.fig,*.pem
+        skip: CHANGELOG,Makefile,*.fig,*.pem,./doc/design-thoughts,./doc/internals
         ignore_words_list: ist,ists,hist,wan,ca,cas,que,ans,te,nd,referer,ot,uint,iif,fo,keep-alives,dosen,ifset,thrid,strack,ba,chck,hel,unx,mor