gh-35304: HTML documentation: Add copy buttons to code blocks
    
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### 📚 Description
A copy button is added to the top right of each code block in the HTML
documentation.

Key added functionality, compared to just selecting the code and copy-
pasting it to a Sage session:
- it's just one click
- the output of doctests is omitted
- comments (in particular the many new `# optional - ...` comments) are
stripped away

Example: https://deploy-preview-35304--sagemath-tobias.netlify.app/refer
ence/discrete_geometry/sage/geometry/polyhedron/base7.html#sage.geometry
.polyhedron.base7.Polyhedron_base7.volume

Resolves #35239

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### 📝 Checklist

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### ⌛ Dependencies
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URL: #35304
Reported by: Matthias Köppe
Reviewer(s): Kwankyu Lee, Tobias Diez

Author: Release Manager

build/pkgs/sagemath_doc_html/dependencies

@@ -1,4 +1,4 @@
-sagelib sphinx pplpy_doc | $(SAGERUNTIME) maxima networkx scipy sympy matplotlib pillow mathjax mpmath ipykernel jupyter_client conway_polynomials tachyon jmol ipywidgets jupyter_sphinx sage_docbuild elliptic_curves furo fpylll
+sagelib sphinx sphinx_copybutton pplpy_doc | $(SAGERUNTIME) maxima networkx scipy sympy matplotlib pillow mathjax mpmath ipykernel jupyter_client conway_polynomials tachyon jmol ipywidgets jupyter_sphinx sage_docbuild elliptic_curves furo fpylll
 
 # Building the documentation has many dependencies, because all
 # documented modules are imported and because we use matplotlib to

build/pkgs/sphinx_copybutton/SPKG.rst

@@ -0,0 +1,18 @@
+sphinx_copybutton: Add a copy button to each of your code cells.
+================================================================
+
+Description
+-----------
+
+Add a copy button to each of your code cells.
+
+License
+-------
+
+MIT License
+
+Upstream Contact
+----------------
+
+https://pypi.org/project/sphinx-copybutton/
+

build/pkgs/sphinx_copybutton/checksums.ini

@@ -0,0 +1,5 @@
+tarball=sphinx_copybutton-VERSION-py3-none-any.whl
+sha1=329d24e382c80fbb39aba221479f3ca3f566d19c
+md5=1181950a5fd59160f7ec588e805d986f
+cksum=142562474
+upstream_url=https://pypi.io/packages/py3/s/sphinx_copybutton/sphinx_copybutton-VERSION-py3-none-any.whl

build/pkgs/sphinx_copybutton/dependencies

@@ -0,0 +1,4 @@
+$(PYTHON) sphinx | $(PYTHON_TOOLCHAIN)
+
+----------
+All lines of this file are ignored except the first.

build/pkgs/sphinx_copybutton/install-requires.txt

@@ -0,0 +1 @@
+sphinx-copybutton

build/pkgs/sphinx_copybutton/package-version.txt

@@ -0,0 +1 @@
+0.5.1

build/pkgs/sphinx_copybutton/type

@@ -0,0 +1 @@
+standard

configure.ac

@@ -496,7 +496,7 @@ AC_ARG_ENABLE([doc],
   AS_HELP_STRING([--disable-doc],
                  [disable build of the Sage documentation and packages depending on it]), [
     dnl Disable packages needed for docbuilding
-    for pkg in sage_docbuild alabaster babel snowballstemmer imagesize sphinx sphinxcontrib_devhelp sphinxcontrib_jsmath sphinxcontrib_serializinghtml sphinxcontrib_applehelp sphinxcontrib_htmlhelp sphinxcontrib_qthelp sphinxcontrib_websupport jupyter_sphinx furo; do
+    for pkg in sage_docbuild alabaster babel snowballstemmer imagesize sphinx sphinxcontrib_devhelp sphinxcontrib_jsmath sphinxcontrib_serializinghtml sphinxcontrib_applehelp sphinxcontrib_htmlhelp sphinxcontrib_qthelp sphinxcontrib_websupport jupyter_sphinx furo sphinx_copybutton; do
       AS_VAR_SET([SAGE_ENABLE_$pkg], [$enableval])
     done
     AS_VAR_IF([enableval], [no], [dnl Disable the docbuild by disabling the install tree for documentation

src/sage_docbuild/conf.py

@@ -46,6 +46,7 @@
     'sphinx.ext.todo',
     'sphinx.ext.extlinks',
     'sphinx.ext.mathjax',
+    'sphinx_copybutton',
     'IPython.sphinxext.ipython_directive',
     'matplotlib.sphinxext.plot_directive',
     'jupyter_sphinx',
@@ -255,6 +256,12 @@ def set_intersphinx_mappings(app, config):
 # By default document are not master.
 multidocs_is_master = True
 
+# https://sphinx-copybutton.readthedocs.io/en/latest/use.html
+copybutton_prompt_text = r"sage: |[.][.][.][.]: |\$ "
+copybutton_prompt_is_regexp = True
+copybutton_exclude = '.linenos, .c1'  # exclude single comments (in particular, # optional!)
+copybutton_only_copy_prompt_lines = True
+
 # Options for HTML output
 # -----------------------