fix: use file move in stead of read

[watermarkhu]

Allows the plugin to be used with mkdocs-same-dir. Alternative fix proposed in also oprypin/mkdocs-same-dir#13. But this is a good change nonetheless.

[landmaj]

Thanks for the change, however I think it should be fixed in mkdocs-same-dir. The code as it is now works and I would rather not change it to something untested if it does not fix or improve anything. And there is nothing to fix. In MkDocs 1.6 (which is a requirement of this plugin) files can be stored in memory, as specified here. If not with this one, than at some point mkdocs-same-dir will break with some other plugin.

Since MkDocs 1.6 a file may alternatively be stored in memory - content_string/content_bytes.

Then src_dir and abs_src_path will remain None. content_bytes/content_string need to be written to, or populated through the content argument in the constructor.

But src_uri is still populated for such files as well! The virtual file pretends as if it originated from that path in the docs directory, and other values are derived.