new header anchor links mode (using #)

[vincent-peugnet]

Add a second mode that add a clickable # after the heading text.

%<element>?headeranchor=(0|1)%

New syntax:

%<element>?headeranchor=(0|1|2)%

Where mode 2 is the clickable #

[n-peugnet]

The <a> surrounding th # should have a class for easy styling.

For instance, Sphinx adds a headerlink class to these links (inspect https://www.sphinx-doc.org/en/master/)

It is also a good idea not to add whitespace between the <a> and the rest of the title, to avoid this kind of issue : readthedocs/sphinx_rtd_theme#1412
Another possibility to avoid it is to use a non-breaking space   to separate it from the rest of the title but then you have less styling flexibility.

[vincent-peugnet]

I just note another option we've tested:

<h1>Titre un peu long oh yeah<span class="headerlink nbsp">&nbsp;</span><a href="#titre" class="headerlink">#</a></h1>

Putting the   inside a <span> tag allow advanced users to remove it using CSS easily.