HTML5 permalinks are not permanent if section header starts with number

In pip Changelog slugs in html anchors are not permanently pointed to corresponding version. Instead, they are incremental position numbers, which start with #id1, so when new version of pip is released all anchors shift and start to point to a different version.

To Reproduce

#!/bin/bash

DOCDIR=testnumslug

rm -rf $DOCDIR
mkdir $DOCDIR

cat <<EOF > $DOCDIR/index.rst
Hi
==

1.2.0
-----

1.1.0
-----

1.0.0
-----
EOF

# Application error:
# config directory doesn't contain a conf.py file (testnumslug)
touch $DOCDIR/conf.py

sphinx-build $DOCDIR $DOCDIR/_html

echo -e "\n-----\n"

grep -R 'Permalink' $DOCDIR/_html/index.html

This gives the output.

<h1>Hi<a class="headerlink" href="#hi" title="Permalink to this headline">¶</a></h1>
<h2>1.2.0<a class="headerlink" href="#id1" title="Permalink to this headline">¶</a></h2>
<h2>1.1.0<a class="headerlink" href="#id2" title="Permalink to this headline">¶</a></h2>
<h2>1.0.0<a class="headerlink" href="#id3" title="Permalink to this headline">¶</a></h2>

Expected behavior

<h1>Hi<a class="headerlink" href="#hi" title="Permalink to this headline">¶</a></h1>
<h2>1.2.0<a class="headerlink" href="#1.2.0" title="Permalink to this headline">¶</a></h2>
<h2>1.1.0<a class="headerlink" href="#1.1.0" title="Permalink to this headline">¶</a></h2>
<h2>1.0.0<a class="headerlink" href="#1.0.0" title="Permalink to this headline">¶</a></h2>

Environment info

• Python version: 3.9.1
• Sphinx version: 3.2.1

Additional context

• https://github.com/pypa/pip/issues/8152

Start by running the provided shell reproduction with Sphinx 3.2.1 and inspect the generated testnumslug/_html/index.html. Locate the HTML5 permalink generation path and add a regression test for section headings beginning with numbers; done means numeric headings receive stable anchors based on their text rather than shifting id1/id2 values.