Include Page "source" files in watchlist when using --server

[NiklasRosenstein]

When using the --server option, Pydoc-Markdown watches a list of files for changes and re-triggers the render process if any of the files change. However, the files defined in a Page.source (as can be configured in the MkDocs and Hugo renderers) is not included in that list. Thus editing the source file does not re-trigger the render operation