Embed Docsy version in <meta> tag of generated pages

[chalin]

• This would be similar to what is done by Hugo via <meta name="generator" content="Hugo 0.126.1">.
• We might want to comply to generating the tag in the same scope as Hugo?

Here's a snippet that from https://github.com/chalin/docsy/blob/333c2f8d41725d5a38e41b1ab9b2ea58622bc37d/layouts/partials/head-css.html:

{{ if ... -}}
    <meta name="docsy-info" content="{{ $msg }}">
{{ end -}}