docs: Fix path to GHC API docs in index.html

In the make bindists we generate documentation in docs/ghc-<VER> but the
hadrian bindists generate docs/ghc/ so the path to the GHC API docs was
wrong in the index.html file.

Rather than make the hadrian and make bindists the same it was easier to
assume that if you're using the mkDocs script that you're using hadrian
bindists.

Fixes #21509

.gitignore

@@ -112,7 +112,6 @@ _darcs/
 /distrib/configure.ac
 /distrib/ghc.iss
 /docs/man
-/docs/index.html
 /docs/users_guide/.log
 /docs/users_guide/users_guide
 /docs/users_guide/ghc.1

configure.ac

@@ -1222,7 +1222,6 @@ AC_CONFIG_FILES(
   libraries/libiserv/libiserv.cabal
   libraries/template-haskell/template-haskell.cabal
   docs/users_guide/ghc_config.py
-  docs/index.html
   libraries/prologue.txt
   distrib/configure.ac
 ])

docs/index.html.in → docs/index.html

@@ -39,7 +39,7 @@
 
       <LI>
         <P>
-          <B><A HREF="libraries/ghc-@ProjectVersionMunged@/index.html">GHC API</A></B>
+          <B><A HREF="libraries/ghc/index.html">GHC API</A></B>
         </P>
         <P>
           Documentation for the GHC API.