Consider vendoring the entire docsy template

While debugging https://github.com/apache/parquet-site/issues/159 it was not obvious to me where the overall site templates were coming from. This may just be my own lack of experience with Hugo/Docsy but as we go and make more extensive modifications to the templates, I think it would help to make it clearer where the source comes from and how to change it

Also, I think reducing the amount of third-party content not under (our) source control, that is used to build the site would make it less likely we hit issues such as https://github.com/apache/parquet-site/issues/159 (due to third-party library being updated)

Here is a PR with this this idea
- https://github.com/apache/parquet-site/pull/160

Start by reviewing how the Hugo/Docsy templates are currently sourced and then read the proposed approach in parquet-site pull request 160. Done means the relevant Docsy template content is vendored under project control and contributors can clearly find its source and understand how to modify it.

Written by the indexing model from the issue text.