Fix slashes in versioned docs

[QuantumDancer]

While the versioned docs (see #553 ) generally seem to work, I found to minor issues:

1. The generated HTML code has unicode slashes (/) in link tags, while it has normal slashes for image tags (see the source code of https://alu-schumacher.github.io/AUDITOR//edge/):

<a href="https:&#x2F;&#x2F;alu-schumacher.github.io&#x2F;AUDITOR&#x2F;&#x2F;edge&#x2F;">
    <div class="logo">
        <img src="https://alu-schumacher.github.io/AUDITOR//edge/white_transparent_background.png" alt="logo">
         UDITOR
    </div>
</a>

Not sure if this is a problem or if all browser can handle addresses like https://alu-schumacher.github.io/AUDITOR//edge/

Also not sure, why this happens for link tags but not for image ones. Maybe some sort of URL encoding going in in the zola build? This also occurs for the old version docs that have been generated on a local machine (e.g. view-source:https://alu-schumacher.github.io/AUDITOR/v0.3.1/)

2. The GITHUB_PAGES_URL already contains a trailing slash. Right now be build the base URL with ${{ env.GITHUB_PAGES_URL }}/${{ env.RELEASE_VERSION }}, which should be changed to ${{ env.GITHUB_PAGES_URL }}${{ env.RELEASE_VERSION }}