Issue #27 - proposal to shift anchors

[mathieucarbou]

proposal to shift anchors to not hide the top of the scrolled-to sections.

This fix changes:

<h3 id="code-cache-code-elements”>
    <a class="anchor" href="#code-cache-code-elements"></a>
    <code>&lt;cache&gt;</code> elements
</h3>

to:

<h3>
    <a class="anchor" name="code-cache-code-elements" id="code-cache-code-elements"></a>
    <code>&lt;cache&gt;</code> elements
</h3>

and also applies a CSS to shift the anchor <a name...> 70px above the section, to account for the floating banner.

This solves issue #27 and is compatible with normal anchors like we have in the documentation page at http://localhost:4000/documentation/#historical_versions

The real fix would be to control the asciidoc generation though...

[mathieucarbou]

@ljacomet : rebased!

[boyusun]

I just realize this pr is the same as the pr I just created: #92. If possible, please refer to my PR.