Fix doc build for Sphinx 4.0

Use app.add_css_file() to register krb5.css if possible (it was added
in Sphinx 1.8), since the old name app.add_stylesheet() was removed in
Sphinx 4.0.

Use the highlight directive instead of the highlightlang directive,
which was removed in Sphinx 4.0.

Remove two duplicate table of contents entries to fix warnings.

In the Github Actions configuration, add a second doc build using the
newest version of Sphinx.

(cherry picked from commit 3fa40a32e22cb9de91fa1d18deddcba446515855)

ticket: 9006
version_fixed: 1.19.2

8 files changed, 27 insertions, 8 deletions

diff --git a/.github/workflows/doc.yml b/.github/workflows/doc.yml
index 292df4c..75f467c 100644
--- a/.github/workflows/doc.yml
+++ b/.github/workflows/doc.yml
@@ -5,7 +5,7 @@ on:
     pull_request: {paths: [doc/**, src/doc/*, src/include/krb5/krb5.hin, .github/workflows/doc.yml]}
 
 jobs:
-    doc:
+    doc-older-sphinx:
         runs-on: ubuntu-18.04
         steps:
             - name: Checkout repository
@@ -19,6 +19,20 @@ jobs:
               run: |
                 cd src/doc
                 make -f Makefile.in SPHINX_ARGS=-W htmlsrc
+    doc-newest-sphinx:
+        runs-on: ubuntu-18.04
+        steps:
+            - name: Checkout repository
+              uses: actions/checkout@v1
+            - name: Linux setup
+              run: |
+                sudo apt-get update -qq
+                sudo apt-get install -y doxygen python3-lxml python3-pip
+                pip3 install Cheetah3 sphinx
+            - name: Build documentation
+              run: |
+                cd src/doc
+                make -f Makefile.in SPHINX_ARGS=-W htmlsrc
             - name: Upload HTML
               uses: actions/upload-artifact@v2
               with:
diff --git a/doc/appdev/refs/macros/index.rst b/doc/appdev/refs/macros/index.rst
index cebb664..5542d98 100644
--- a/doc/appdev/refs/macros/index.rst
+++ b/doc/appdev/refs/macros/index.rst
@@ -55,7 +55,6 @@ Public
    ENCTYPE_DES3_CBC_RAW.rst
    ENCTYPE_DES3_CBC_SHA.rst
    ENCTYPE_DES3_CBC_SHA1.rst
-   ENCTYPE_DES3_CBC_SHA1.rst
    ENCTYPE_DES_CBC_CRC.rst
    ENCTYPE_DES_CBC_MD4.rst
    ENCTYPE_DES_CBC_MD5.rst
diff --git a/doc/appdev/refs/types/index.rst b/doc/appdev/refs/types/index.rst
index dc414cf..d8d2a8f 100644
--- a/doc/appdev/refs/types/index.rst
+++ b/doc/appdev/refs/types/index.rst
@@ -62,7 +62,6 @@ Public
    krb5_preauthtype.rst
    krb5_principal.rst
    krb5_principal_data.rst
-   krb5_const_principal.rst
    krb5_prompt.rst
    krb5_prompt_type.rst
    krb5_prompter_fct.rst
diff --git a/doc/appdev/refs/types/krb5_int32.rst b/doc/appdev/refs/types/krb5_int32.rst
index 2bc914b..28baafa 100644
--- a/doc/appdev/refs/types/krb5_int32.rst
+++ b/doc/appdev/refs/types/krb5_int32.rst
@@ -1,4 +1,4 @@
-.. highlightlang:: c
+.. highlight:: c
 
 .. _krb5-int32-struct:
 
diff --git a/doc/appdev/refs/types/krb5_ui_4.rst b/doc/appdev/refs/types/krb5_ui_4.rst
index de79baf..73eb38c 100644
--- a/doc/appdev/refs/types/krb5_ui_4.rst
+++ b/doc/appdev/refs/types/krb5_ui_4.rst
@@ -1,4 +1,4 @@
-.. highlightlang:: c
+.. highlight:: c
 
 .. _krb5-ui4-struct:
 
diff --git a/doc/conf.py b/doc/conf.py
index 543202b..14158ae 100644
--- a/doc/conf.py
+++ b/doc/conf.py
@@ -98,8 +98,15 @@ pygments_style = 'sphinx'
 
 # -- Options for HTML output ---------------------------------------------------
 
+# When we can rely on Sphinx 1.8 (released Sep 2018) we can just set:
+#   html_css_files = ['kerb.css']
+# But in the meantime, we add this file using either a way that works
+# after 1.8 or a way that works before 4.0.
 def setup(app):
-    app.add_stylesheet('kerb.css')
+    if callable(getattr(app, 'add_css_file', None)):
+        app.add_css_file('kerb.css')
+    else:
+        app.add_stylesheet('kerb.css')
 
 # The theme to use for HTML and HTML Help pages.  See the documentation for
 # a list of builtin themes.
diff --git a/doc/tools/define_document.tmpl b/doc/tools/define_document.tmpl
index ca56d86..8e74dc3 100644
--- a/doc/tools/define_document.tmpl
+++ b/doc/tools/define_document.tmpl
@@ -1,4 +1,4 @@
-.. highlightlang:: c
+.. highlight:: c
 
 .. $composite.macro_reference($composite.name):
 
diff --git a/doc/tools/type_document.tmpl b/doc/tools/type_document.tmpl
index 5987fa7..11aafb8 100644
--- a/doc/tools/type_document.tmpl
+++ b/doc/tools/type_document.tmpl
@@ -1,4 +1,4 @@
-.. highlightlang:: c
+.. highlight:: c
 
 .. $composite.struct_reference($composite.name):