small html theme tweaks to unclutter top of page. move version string to navigation bar

<div role="navigation" aria-label="breadcrumbs navigation">

  <div style="text-align: center; margin-bottom: -1.5em; display: block"><b>LAMMPS</b> {{ version }}</div>

  <ul class="wy-breadcrumbs">

    <li><a href="{{ pathto(master_doc) }}">Docs</a> &raquo;</li>

      {% for doc in parents %}

        {% endif %}

      </li>

  </ul>

  <hr/>

  <hr width="100%"/>

  {% if next or prev %}

    <div class="rst-footer-buttons" style="margin-bottom: 1em" role="navigation" aria-label="footer navigation">

      {% if next %}

        {% if logo %}

          {# Not strictly valid HTML, but it's the only way to display/scale it properly, without weird scripting or heaps of work #}

          <img src="{{ pathto('_static/' + logo, 1) }}" class="logo" />

          <div><b>{{ version }}</b></div>

        {% endif %}

        </a>